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Forensic science 

Forensic science
Forensic science

Jonathan P. Wyatt

, Tim Squires

, Guy Norfolk

, and Jason Payne-James

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PRINTED FROM OXFORD MEDICINE ONLINE ( © Oxford University Press, 2016. All Rights Reserved. Under the terms of the licence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Medicine Online for personal use (for details see Privacy Policy and Legal Notice).

date: 21 October 2019

  • Introduction to forensic science [link]

  • Crime scene management [link]

  • Crime scene investigation [link]

  • Locards principle [link]

  • Identification: matching and uniqueness [link]

  • Daubert [link]

  • Trace evidence: background [link]

  • Trace evidence: glass [link]

  • Trace evidence: paint [link]

  • Trace evidence: hairs and fibres [link]

  • Fingerprints [link]

  • Forensic analysis of DNA [link]

  • Interpretation of DNA analysis [link]

  • National DNA database [link]

  • Exonerating the innocent [link]

  • Forensic biology [link]

  • Source of blood [link]

  • Blood pattern analysis [link]

  • Forensic identification of semen [link]

  • Forensic identification of other fluids [link]

  • Forensic anthropology [link]

  • Forensic archaeology [link]

  • Forensic entomology [link]

  • Environmental forensics [link]

  • Document analysis [link]

  • Forensic miscellany [link]

  • Fire: background [link]

  • Fire prevention and safety [link]

  • Behaviour of fire indoors [link]

  • Behaviour of fire outdoors [link]

  • Fire: investigation [link]

  • Explosions and explosives [link]

  • Explosion investigation [link]

  • The context effect and scientific evidence [link]

  • Forensic statistics [link]

  • The prosecutors fallacy [link]

Introduction to forensic science


It is not the purpose of this book to provide instruction in the forensic sciences. However, it cannot be over-emphasized that forensic investigation is a multidisciplinary team effort and it is imperative that individual members of the team have at least an elementary understanding of the roles of experts in other disciplines. Not only does this ensure that evidence gathering, analysis, and interpretation is carried out efficiently and effectively, but it also goes some way to ensuring that expert witnesses in a court setting are, indeed, experts in the subjects to which they speak.


What is meant by the term ‘forensic science’? Arguably, there is no such thing, if the term is intended to refer to a separate and clearly defined discipline. Forensic science is really an umbrella description relating to the practice of any science within a legal context. It is worth emphasizing that good forensic science is, therefore, nothing other than good science applied to a forensic context.

Why then include a chapter which purports to cover the ‘forensic sciences’? This chapter simply presents an overview of the scientific disciplines which to a greater-or-lesser extent are often called upon in forensic practice. It is not claimed to be inclusive of all the forensic sciences, nor suggested that it describes any of them in any depth.

Key features

In general terms, forensic science means the application of rigorous scientific methodology and validated laboratory practices and techniques in order to generate results which can be used as evidence.

Key concepts relating to evidence include:

  • Protection

  • Recording

  • Collecting/recovery

  • Preserving (especially avoiding contamination)

  • Analysing

  • Interpreting

  • Evaluation

  • Presenting.

It should be noted that errors at any stage of this process may cause an otherwise strong case to collapse. Not only does this lead to a miscarriage of justice in that individual instance, it also results in an erosion of the public's confidence in the forensic sciences.

The importance of adhering strictly to the SOPs and operating guidelines in forensic laboratories cannot be over-estimated.

Forensic science and the court

How scientific evidence is presented in the court room is an oft neglected part of this chain. It should be clear that no matter how good the science, if it is poorly presented to a lay audience (i.e. the jury) it will fail to convince. Yet just as the scientist or other expert witness has an obligation to present their evidence clearly with precision and accuracy, this book is also intended to assist the lawyer in his or her ability to question and cross-examine such evidence.

Forensic science agencies and organizations

  • Forensic Science Society

  • American Academy of Forensic Sciences (AAFS)

  • The Australian and New Zealand Forensic Science Society (ANZFSS)

  • European Network of Forensic Science Institutes (ENFSI)

  • Scottish Police Services Authority (SPSA) Forensic Services.

Crime scene management

What is a crime scene?

Although this might seem obvious, it is in fact one of the most important questions to be asked. A crime scene is defined whenever an illegal act is suspected to have taken place. The crucial issue is to define the physical limits of the crime scene—this is important as it should contain the maximum possible quantity (and quality) of evidence, whilst occupying a volume in which it is practical to search. Depending on the type of offence alleged to have been committed, this might be a room, a house, or even a whole street. It should be obvious that the wider the crime scene is defined, the greater the disruption to the public and this has to be balanced against the need to secure ‘all’ the evidence. The crime scene can also include areas in which evidence relating to the alleged offence might be located, even if this is not where the offence occurred. There are also circumstances in which the decision as to when a situation becomes a ‘crime scene’ is important. If a child has been reported to be late home from school, when (and where) does this become a ‘crime scene’? It is important that law enforcement officers are able to refer to clear guidelines and policies when confronted by such circumstances.

Priorities at a crime scene

These are:

  • Preservation of life (of victims, the public, and investigators)

  • Preservation of the scene

  • Gathering of evidence.

Law enforcement officers arriving at an incident are often confronted by a confusion of events and should always treat a scene as a scene of crime until confirmed otherwise. This should include calling out a forensic pathologist if a death is considered to be suspicious.

It is advisable to err on the side of caution and assume that a scene is a crime scene in order to protect evidence. Downgrading a scene (e.g. to an ‘accident’ or ‘sudden natural death’) is straightforward and costs nothing other than minor inconvenience. It is likely that evidence will be lost if it is not treated as a crime scene initially—with significant negative consequences both for the investigation and the reputation of the law enforcement agencies.

A certain amount of ‘common sense’ must be exercised in the case of large-scale, ‘public’ crime scenes. For example, in the case of the Lockerbie air disaster (21 December 1988), bodies were removed from public view at the earliest opportunity.

Some crime scenes are difficult to define in physical or geographical terms—notably, Internet-based crime such as child pornography or fraudulent (financial or identity) activity.

Health and safety

All crime scenes are potentially hazardous. Risk cannot be removed entirely, but efforts must be made to carry out a rigorous risk assessment and implement appropriate precautions. Advice from other agencies (such as fire or maritime rescue services) should be sought if relevant.

Approach to the crime scene

Crime scenes are rarely static and the quality of evidence deteriorates with time. Steps should be implemented to reduce the rate and extent of deterioration including:

  • Cordons

  • Common approach paths

  • Control of scene.

Typically, a crime scene requires an inner cordon, an outer cordon with a single entry point to both, and a single pathway to the focus of the incident. Although the primary concern at this stage should be the preservation of evidence, consideration to unauthorized viewing (particularly if outdoors) should be made and, if necessary, a tent or screen erected. Press interest in serious incidents is significant and although visual access must be limited, it important to handle this with sensitivity (especially as relatives, friends, and neighbours of victims may be present). A Scene Entry Log should be maintained as quickly as possible and any possible sources of contamination (e.g. blankets which may have been put over a deceased) kept for examination at a later date.

Modern practice requires that whenever there is a crime scene, there should be a crime scene manager (CSM, who may be the Senior Investigating Officer, SIO). This CSM takes charge of the scene, including controlling and recording access of other experts. Forensic pathologists and forensic scientists, no matter how experienced, must act in accordance with the crime scene manager. The only exceptions are emergency medical staff treating injured people who remain at the scene.

If the crime involves several scenes, then a crime scene coordinator is appointed. Crucial to this role is to ensure there no cross-contamination between scenes (which may, of course, involve the same experts).

Recovery of evidence

Recovering evidence from a crime scene is the responsibility of a wide range of experts. In certain types of incident it is likely that law enforcement officers will work in close collaboration with other statutory agencies such as aviation, marine, customs, fire, and environmental (pollution) investigators or organizations such as those with a remit to protect wildlife:

  • Civil Aviation Authority (CAA)

  • Air Accidents Investigation Branch (AAIB)

  • Marine Accident Investigation Branch (MAIB)

  • HM Revenue and Customs (HRRC)

  • Department for Environment Food and Rural Affairs (DEFRA)

  • Royal Society for the Prevention of Cruelty to Animals (RSPCA)

  • Royal Society for the Protection of Birds (RSPB).

Crime scene investigation


Crime scene investigation should attempt to address each of the following issues:

  • Has a crime been committed?

  • How/when/where was it committed?

  • What is the identity of the perpetrator?

Note that a scene or incident might involve one or more designations. For example, a scene of a sudden death might prove to be drug-related and as such would have potential implications for a prosecution for supply of drugs or even manslaughter/culpable homicide.

If in doubt, it is important to treat a scene as a crime scene to avoid the risk of losing vital evidence. A scene can always be downgraded at a later stage, once investigators are confident that no crime has been committed.

Accident or crime scene?


Although initially clearly a scene having all the characteristics of a major accident, investigation into the Lockerbie air disaster (21 December 1988) revealed that it resulted from a criminal act. More recently, railway ‘accidents’, have resulted in criminal prosecutions on the basis of corporate manslaughter legislation.

Corporate manslaughter

The first UK case of corporate manslaughter involved OLL Ltd. Peter Kite, the Managing Director was jailed for 2 years (after appeal) in 1994 following the deaths of four teenagers in a canoeing incident at an outdoor activity centre at Lyme Regis, Dorset.

Crime scene recording

It is important that crime scene investigation includes crime scene recording. This should involve the use of traditional techniques, such as photography (always employing a scale for reference) and scale drawings (using computer software), but should also make use of crime scene management and reconstruction software. The ability to create a virtual scene is a valuable tool to assist interpretation and evaluation of evidence and/or events at a later date.

Crime scene evidence

The question of whether the evidence is believed and ‘proves’ the case as presented by the prosecution is, of course, a matter for the jury or judge to decide. Prior to that consideration, it is imperative that the integrity of the prosecution's evidence is preserved and can be demonstrated to the court. A chain of continuity should exist for each item of evidence, providing a complete audit trail accounting for all movement, handling, storage, and analysis from the scene to the mortuary/laboratory and, if appropriate, presentation in court. It should be possible to scrutinize the chain at any point to eliminate the possibility of contamination, damage, or deterioration having occurred which might be prejudicial to the case. All Health and Safety procedures associated with the handling, storage, and analysis of the item or material should also be documented so any adverse effects are known and can be described to the court.

Locard's principle

Edmond Locard (1877–1966) is often regarded as the father of modern evidence-based forensic science. The famous dictum summarizing his work:

‘Every contact leaves a trace’

is axiomatic with the basic principle of forensic science. In purely practical terms, this simply means that it is always worth looking for evidence wherever a ‘contact’ is suspected. Remember that although this necessitates thorough and comprehensive police work at every crime scene, it says very little about the evidential significance of whatever might be found.

Significance of evidence

Note that in order to acquire evidential significance the contact does not need to be directly between two people. Suppose ‘A’ is alleged to have committed a crime against ‘B’. If there has been physical contact between A and B then it is possible that there has been a direct transfer between the two: for example, semen, blood, hair, or DNA from one individual is transferred to the other (perpetrator to victim and/or victim to perpetrator). Matching the transferred sample to the source establishes contact. However, evidence of contact does not establish a reason for the contact and does not, therefore, ‘prove’ the prosecution's case. The defence might argue that there is an alternative and innocent explanation, that the sample has been transferred by an alternative mechanism (such as third-party contact), or that the sample has been contaminated by poor collecting, handling, storage, or analysis.

Alternatively, there might be a transfer from a common source which establishes indirect contact between individuals ‘A’ and ‘B’: the presence of a particular pollen, dust particles, fragments of a particular glass or fibres can provide evidence that ‘A’ and ‘B’ were ‘at the same place’ or, more generally, exposed to the same source of the sample. Again, note that this does not necessarily say anything about when this exposure occurred or why.

Applications in alleged sexual assault

Both the profound power and limitations of Locard's principle are manifest in allegations of rape. If semen is identified in a sample obtained from the complainant, then the science can prove (subject to the usual arguments about transfer, contamination, etc.) that sexual intercourse occurred. If, however, the suspect does not deny that sexual intercourse occurred, then the evidential significance of the science is minimal (the issue being whether there was consent to the act, not whether the act took place). Even if there is conclusive scientific evidence of contact, an ‘alternative explanation’ may render this evidence irrelevant to the case.

DNA from mosquitoes

Spitaleri and colleagues1 report a case in which DNA matching that of a homicide victim was recovered from the blood meal stain of a mosquito found in the suspect's house. Although the technical aspects of this case are worthy of note, in this context the focus is the importance of the scene investigation and how an apparently innocuous finding (a squashed mosquito) actually held crucial evidence: scientific advances do not replace thorough and comprehensive police work:

  • Note that if no evidence of contact is found, this does not establish that no contact took place.

  • Evidence of contact does not establish guilt and the absence of evidence of contact does not establish innocence.


1 Spitaleri S, Roman C, Di Luise E, et al. (2006). Genotyping of human DNA recovered from mosquitoes found on a crime scene. International Congress Series 1288:574–6. (Progress in Forensic Genetics 11: Proceedings of the 21st International ISFG Congress, Ponta Delgada, September 2005.)

Identification: matching and uniqueness

Basic concepts

In forensic investigation, there is another concept which is just as important as Locard's principle, namely the concept of identification—matching and uniqueness. A forensic sample which is obtained from a crime scene (for example, DNA, a fingerprint, a fibre, a fragment of paint, or a hair) only acquires evidential significance once it can be identified with, or matched to, a reference sample recovered elsewhere or from a suspect.

Unique match

The concept of a unique match is of particular importance in a court room. If a particular witness reports seeing a ‘red car with a white stripe’ leaving a crime scene, it is not sufficient to locate any white striped red car. A successful prosecution requires that the unique vehicle be identified (and proved to be such to the required standard of proof). In this example, a fragment of paint recovered from the scene can (by comparing chemical composition of the fragment to various paint manufacturers' databases) be matched to a particular batch and/or series of vehicles: it is unlikely that it can be matched to a unique vehicle unless a ‘physical fit’ can be achieved. Note that although chemical analysis of the paint fragment may enable investigators to focus on a particular class of car, it is only once a suspect vehicle is identified that a unique physical fit match is possible.


The extent to which ‘item A matches item B’ is often a question of magnification. Tearing a piece of paper into two pieces well illustrates this point. At a distance, the detail of the tear may be insufficient to distinguish unique features: the visible detail is not specific enough to enable a unique match to be made. As the torn edges are examined at higher magnifications, the required level of detail is exhibited and a match can be made with confidence. But note that at even higher resolutions, the ‘perfect match’ will be lost as minute tags and fragments are displaced and lost and the pattern is distorted. The skill of the forensic scientist is not only to know where and when to start looking but also to know when and where to stop. Irrelevant detail can be as misleading as insufficient information.

In a court of law, the expert should be cautious about matching an injury with a unique weapon. In most cases, the correct terminology to use is that an injury ‘is consistent with’ a type of weapon.

Knife injuries

For the forensic pathologist, it is important to appreciate that injuries can rarely be matched to a unique weapon such as a knife. The best that can usually be achieved is that the medical expert who examines the injury is able to state that the findings are ‘consistent with’ a particular type of weapon. This may include, for example, a knife produced as evidence in court. To state that the injury was caused by one particular knife requires more—an obvious example being if the tip breaks off during an attack and remains in the body. The tip can then be matched uniquely to the knife by a ‘physical fit’ procedure.

Gunshot injuries

Although it might be possible to state that a bullet recovered from an injury was fired from one particular gun, the injury itself can usually only be matched to a class or type of weapon.

Certain injuries are more likely to reveal information about the type of weapon (see Forensic science [link]).

The Ruxton case

Double murder

The Ruxton case was an early demonstration of the success of forensic science and led to an increased professional and public trust in the technological advances being employed in the court room. Ruxton was an Indian-born doctor living near the border between England and Scotland. On 14 September 1935, he murdered his wife, Isabella, and her maid, Mary Rogerson, mutilating their bodies and scattering the remains in an attempt to render them unidentifiable.

Pioneering techniques

The discovery of human remains under a bridge in Scotland was the chance event which initiated the investigation. A forensic team led by forensic pathologist John Glaister and the anatomist James Couper Brash used pioneering techniques of photographic superimposition and managed to identify the bodies and produced the key evidence in the successful prosecution.1 Whether this level of ‘match’ would be accepted by a court today is open to debate.


Dr Ruxton's trial took place in March 1936 and lasted 11 days. He was found guilty and sentenced to be hanged to death. He admitted guilt before his execution.


1 Glaister J and Couper Brash J (1937). Medico-Legal Aspects of the Ruxton Case. Edinburgh: Livingstone.


The Daubert test

The so-called ‘Daubert’ test was introduced in the US courts as a way of regulating the admissibility of scientific evidence. The test requires that the two conditions of relevance and reliability are satisfied before expert scientific evidence can be admitted.

Relevancy test

The relevancy test simply requires that the evidence to be submitted is relevant to the issues of the case. This prevents irrelevant science being produced which, no matter how rigorous or persuasive it is, is simply irrelevant to the questions before the Court. This prevents juries being misled by irrelevant science.

Reliability test

The reliability test requires that scientific evidence and techniques:

  • Are tested in actual conditions not only in the controlled environment of the laboratory;

  • Have been subject to peer review, publication, and general acceptance within the relevant scientific community;

  • Have a known error rate (which should be close to zero); and,

  • Have standard procedures so they can be reproduced and tested by other experts in different settings.

The Daubert standard has particular relevance to the admissibility of evidence based on low copy number DNA analysis. Note that pseudo-scientific techniques would generally be excluded on the basis of Daubert (although whether they have an investigative role is open to further debate).

Further reading

Daubert v. Merrell Dow Pharmaceuticals, 509 US 579 (1993).

Huber P (1991). Galileo's Revenge: Junk Science in the Courtroom. New York: Basic Books.

Trace evidence: background

The nature of trace evidence

Trace evidence denotes any type of evidence which may be recovered from a scene in small amounts. The presence of trace evidence at a scene depends on at least three factors: how easily it separates from the source; the nature of the contact; and the nature of the surfaces involved. Any material which is capable of being transferred in small amounts from one object or place to another has the potential to be of trace evidential value. For example, soil, pollen, vegetable matter, gunshot residue, explosives, fire accelerants, body fluids, and illegal drugs. Analysis leading to a match with a reference sample may be more informative and discriminatory in some cases than others.


It should be obvious that although the techniques used to analyse trace evidence may produce a match between a sample from a crime scene and a reference sample (e.g. one taken from a suspect) they provide no explanation (exculpatory or otherwise) of how or why that sample was at that location.

Case of Ross Rebagliati

Ross Rebagliati was a Canadian snowboarder who tested positive for tetrahydrocannabinol (THC) following his gold medal winning performance at the Nagano Winter Olympic games in 1998. Rebagliati did not deny the scientific evidence of his sample, accepting that he had inhaled cannabis smoke. His successful defence, however, was that the cannabis smoke was the result of his friends smoking cannabis and that he had inhaled it coincidentally because of his proximity.

The case is a good example of detecting trace evidence which was sufficient to prove that an event took place (i.e. inhalation of cannabis smoke), but insufficient to prove that an offence had occurred.

Trace evidence: glass

The main property used to compare pieces of glass is the refractive index (RI). When light passes from one material to another with a different RI, the light path is bent. Conversely, if the two materials have the same RI, the light travels straight: this lack of bending is exploited in the most common technique used to assess the RI.

Glass refractive index measurer (GRIM)

This involves placing a small piece of the glass sample in a depression containing a transparent oil on a microscope slide. The RI of the oil can be changed as its temperature changes. At one temperature, the RI of the oil will equal the RI of the glass. Since there is now no bending of the light at the interface between the glass and the oil at this temperature, the light travelling through the whole assembly is unaffected. Knowing the RI of the oil at all temperatures gives the RI of the glass, which can then be identified from reference databases.

Density of glass is also a useful property in identifying the type or source of a sample of glass.

Physical fit matching of fragments of glass can be useful in, for example, a ‘hit and run’ incident. Fragments recovered at the scene may be matched to those remaining in a headlight on a suspected vehicle. In order to have evidential significance; however, the prosecution should be confident about the ‘uniqueness’ of the claimed match.

Trace evidence: paint

The nature of paint

The features of paint are physical (colour, texture, layers) and chemical (ingredients). Tiny fragments (<1mm) of paint from multilayered sources, such as car bodywork, can be embedded in a resin and sliced through to show the layers of primer, undercoats, and top coats. As with many items that may look the same colour under normal lighting, some items may look very different in other lighting conditions such as UV or monochromatic sources.


This is also used in paint examination, except that in contrast to fibre analysis, which uses measurement of the light transmitted through the fibre, paint analysis uses reflected light (reflectance spectrophotometry). Paint analysis is similar to other organic chemistry, as most of the pigments used in paints are organic compounds. However, some additives give paints specific qualities such as waterproofing, antifungal, fire, or heat proofing, insulating, or flow modifiers. Many of these are metals, or metal compounds which can be detected using scanning electron microscopy (SEM). An electron beam is used to bump electrons within atoms of the sample right out of the atom. This leaves a gap that other electrons in the atom fill, so they drop into the gap left by the ejected electron. As they do so, they emit energy as X-rays. The amount of energy is characteristic of the jump that the electron makes going from its high orbit around the atom to its new low orbit. The jump is different in different elements and can be used to detect, for example, gunshot residue.

If a suspect source is identified (e.g. a vehicle) it might be possible to achieve a physical fit with a paint fragment.

Case of Malcolm Fairley (1985)

One of the most notable cases in which fragments of paint were of evidential significance was the 1980s case of ‘The Fox’ (Malcolm Fairley). Paint fragments found at a scene were matched to a British Leyland ‘Austin Allegro’ produced during 1973–1975. A vehicle which matched this colour was later identified at a suspect's home address. This evidence was crucial in focusing the investigation—it would not have been sufficient to secure a conviction. Fairley was subsequently convicted of a series of violent sexual assaults and rapes and sentenced to six life terms.

Trace evidence: hairs and fibres

Hairs and fibres can be described by colour and shape. The fibre can be natural (e.g. wool) or man-made (e.g. nylon). The shape (morphology) of a fibre can be established by microscopic analysis. Examining morphology can differentiate human and animal hairs due to species-to-species variations; the scale pattern; the ratio of the width of the central region of the hair to the width of the hair, etc.

Hairs from different body sites have characteristic cross-sections:

  • Head: Caucasian (elliptical), African (oval), and Asian (round).

  • Beard, pubic, eyelash: oval.

There may be considerable variations in hair morphology, even in hairs from one individual.

Colour represents the chemical and physical make-up of the fibre. Colour can be natural (i.e. introduced as the fibre is being made) or as a result of artificial dyeing (note that a person's ‘hair colour’ in vivo, may not be the hair colour as determined by chemical analysis).


This exploits the different absorption and transmission characteristics of light of different materials, due to their component atoms, elements, and ions. Light of different and known wavelengths in the visible spectrum is shone through the fibre and an absorption pattern or wavelength scan produced. Scans produced from a sample fibre can be compared with those of reference fibres of known characteristics. Using these techniques, dyes that appear the same colour to the naked eye can be distinguished because they are made up of different components.

Man-made fibres are usually made from long chains of carbon atoms with various atoms and chemical groups added, giving particular properties. Chemical bonds between carbon and these other atoms (and also itself) each absorb light of a different wavelength in the infrared (IR) region of the electromagnetic spectrum. Each fibre and the textile made from that fibre, can be characterized by its absorption spectrum in the IR region of the electromagnetic spectrum. The most common technique to do this is the Fourier transform infrared spectroscopy (‘FTIR’).

Fibres may also be characterized by Ramen spectroscopy (which looks at vibrational energy of the molecules of the fibres) and their birefringence (which is a function of the fibre's refractive indices).

These techniques have, for the most part, replaced destructive methods of analysing the dye components in fibres, which would have been extracted from the fibre and separated by thin layer chromatography.

Determining the melting point of a fibre as well as pyrolysis-gas chromatography (which thermally breaks samples down in to smaller molecules which are then analysed by gas chromatography and mass spectrometry) may be of relevance in differentiating between man-made fibres. Hairs are also a source of DNA. A complete nuclear cellular DNA profile is potentially available from the cells that comprise the root and mitochondrial DNA can be recovered from hairs that have no roots.


Types of fingerprints

Visible prints—fingerprints made by touching glass or, for example, with an ink-covered finger are clearly visible to the naked eye.

Latent prints—a fingerprint not visible without treatment (e.g. application of powder) is termed a latent print. These consist primarily of perspiration from sweat pores.

Plastic prints—when the finger is pressed into a soft material (e.g. chocolate, clay, fresh paint, etc.) a print is made by creating a negative ridge impression.


In the journal Nature (1880), Henry Faulds (1843–1930) published a paper entitled ‘On the Skin-Furrows of the Hand’. He wrote that ‘when bloody finger-marks or impressions on clay, glass etc., exist, they may lead to the scientific identification of criminals…the pattern was unique’. These recognizable patterns form the basis of the classification system which has been adopted in most English-speaking countries. Sir Edward Henry (1850–1931) based his system on three basic types of fingerprint patterns which had been recognized by Sir Francis Galton (1850–1911), namely loops, arches, and whorls. There are also subtypes of the types, but generally speaking, loops are defined where at least one ridge enters from one side, curves round, and exits at the entry side. Arches have ridges flowing from one side to another, rising in the centre in a wave-like pattern. Whorl classification is complex, due to the subtypes, the most simple of which encompass spiral, circular, and oval ridge patterns. As well as the basic pattern type, ridge characteristics including ridge endings and bifurcations may be seen and used for comparison and identification.

Two axioms form the basis for fingerprint identification:

  • Fingerprints are unique.

  • Fingerprints do not change through life.

Print comparisons

The process of comparing a print obtained from a scene with the comparison print can reach one of the following conclusions:

  • The investigated print is identified as coming from the same donor as the comparison print.

  • The investigated print has insufficient information to enable a conclusion about origin.

  • The investigated print is unfit for identification, but exhibits detail sufficient to exclude specific donors.

  • The investigated print enables the expert to state that the investigated person cannot be excluded as the donor.

  • The comparison print is of insufficient quality (the procedure should be halted and a new comparison print may be obtained).

Minimum point rules

Historically, a match was confirmed on the basis of the minimum number of points rule (or an empirical standard approach). However, there is no scientific or statistical basis for the assertion that a match requires a certain (e.g. 12 or 16) number of ‘identical’ points. Generally, a minimum points rule has now been replaced by a more integrated approach in which the court hears evidence from an expert witness who uses a qualitative and quantitative approach to reach a conclusion.

Human error

In addition to the question marks over the scientific status of fingerprint evidence, the techniques and methods employed are particularly prone to human error. One of the most notable cases in recent years is that of Shirley McKie, a police officer with Strathclyde Police. Experts at the Scottish Criminal Records Office mistakenly identified her fingerprints at a crime scene. She was subsequently charged with perjury and although being found not guilty, the case has become an internationally important landmark in the role of fingerprint evidence in modern forensic investigation. A further example of human error was seen after the Madrid bombings (March 2004), in which the US Federal Bureau of Investigation erroneously matched digital images of latent prints to an Oregon lawyer, Brandon Mayfield.

Although there are many documented cases of a wrong ‘match’, unverified techniques used to obtain (‘lift’) the print and the continuing debate over the ‘uniqueness’ of patterns, the use of fingerprints remains a potentially quick and cheap method of identifying bodies.

Prints from a body

When an unknown body is discovered, consideration should always be given to obtaining fingerprints. These are fragile and should be taken before remains are moved. If the epidermis is ‘de-gloved’ (bodies in water), it is possible to take the print from the ‘glove’. In some circumstances (e.g. extensive burning, mummification) it might be desirable to submit the whole hand to a fingerprint expert for examination and with specialist techniques, retrieval of prints. Prints obtained from a deceased individual should be compared with those stored on databases or with fingerprints taken from personal property once an identity is suspected.

Forensic analysis of DNA

DNA and individuality

Our apparent individuality (e.g. hair and eye colour or predisposition to diseases such as cystic fibrosis) arises because of differences in our genes (regions or loci of DNA which control hereditary characteristics exhibiting genetic variation known as alleles). The combination of the two genes (one inherited from each parent in the form of chromosomes) is the genotype and a person's phenotype is the observable result of the genotype. In a forensic context, these differences are relatively tiny (the variations are limited by the need to maintain the functional ability to code proteins) and are of limited use.

DNA profiling

The basis of DNA (deoxyribonucleic acid) profiling in forensic science is in the areas of non-coding DNA lying between the genes (sometimes referred to as ‘junk’ DNA) which comprises about 98.5% of the genome (the complement of the 46 chromosomes normally present). As these regions have no coding function, the variation is unlimited and the possible combinations are such that individuals are (at least to the order of 1 in billions) unique. The only exception is identical twins in whom both the genotype and the ‘junk’ DNA is identical.

Short tandem repeats

Forensic analysis examines regions of the genome called short tandem repeats which are repeated sequences of DNA (typically 4–6 base pairs in length). The variation (and conversely the uniqueness of the individual) arises because the number of repeats is highly variable (polymorphic) across the population (e.g. the locus known as D18S51 [with the sequence AGAA] has been found to be repeated between 7 and 27 times in different individuals). A heterozygotic individual will have different versions of each loci inheriting one from each parent. A homozygotic individual inherits the same version from both parents.

Polymerase chain reaction (PCR)

PCR is a method of increasing (amplifying) the quantity of a DNA sequence obtained from a sample exponentially. Typically a PCR involves 28 cycles, which increases the original sample by a factor of approximately 270 million. In the UK, current forensic practice is to amplify 10 loci (in the US 13 loci are routinely examined), plus a determination of the sex of the individual from a locus known as amelogenin (based on XX chromosomes for females and XY for males).


The process of ‘typing’ or ‘profiling’ is managed by a computer which produces a graphical representation (an electrophoretogram or electropherogram) usually showing peaks along three colour-coded horizontal axes. The position of the peak along the axis indicates the size (number of repeats). Note that the form of this graphical output is dependent on the software used and more sophisticated packages also provide interpretative analysis. Alongside each peak the number of repeats found at that locus will usually be stated. Note that although the height of each peak in a heterozygous person should be the same as it represents the quantity of DNA, there is significant variation because of the chemistry and dynamics of the amplification process. The height of peaks can also be useful in determining which allele comes from which source in a mixed sample.

Paternity disputes

Paternity disputes can be resolved using DNA analysis. If the alleles of the disputed parent are inconsistent with those inherited by the child, then that individual can be excluded as a biological parent. If the alleles are consistent with the child's alleles then the individual is possibly a biological parent. It is important that reference samples are available from all the potential parents.

Mitochondrial DNA

Mitochondrial DNA is inherited from the mother, so is a useful method of tracing the maternal line. In a forensic context, this genetic material is important because it is more resilient to degradation than nuclear DNA. It mutates at a faster rate than nuclear DNA and has less discriminatory power, because all female members of the family will share the same mitochondrial DNA (ignoring mutation). It can be a useful tool when identifying victims of mass disasters. Analysis of mitochondrial DNA requires the sequencing of two hypervariable regions.

Familial DNA

Familial DNA is based on the theory that family members are more likely to have similar DNA. In 2004, Craig Harman was convicted of manslaughter following a close (but not exact) match between a sample obtained at a crime scene and a family member on the DNA database. In the James Lloyd case (2006), a DNA match was made following the close match of a DNA sample obtained from Lloyd's sister after a driving offence. He had committed rapes and sexual offences in the 1980s.

Low copy number

Low copy number DNA analysis remains controversial. The technique is used when only a very tiny quantity of DNA is recovered—such as from a fingerprint. In order to generate sufficient DNA for analysis, this requires more PCR cycles with the consequence that contaminants and other residual DNA will also be amplified. As techniques to recover more minute quantities of DNA develop, the amount of DNA transferred via innocent contact also increases. This potentially results in a greater number of ‘matches’—but ‘matches’ with little or no evidential significance.

Further reading

Low copy number DNA testing in the Criminal Justice system. Available at Forensic science

Omagh Bombing Trial: R v Hoey [2007] NICC 49.

Interpretation of DNA analysis

Importance of interpretation

The interpretation of DNA analysis is at least as important as the science which underlies it. If two profiles (e.g. one from a crime scene, the other from a suspect or a database) have the same alleles at all of the profiled loci, then the samples ‘match’—this means that both samples could have originated from the same source. The random match probability is the probability of obtaining that match if the samples did not come from the same source. This is not the same as the probability that the suspect is innocent.

If the two profiles do not match, then the two samples do not come from the same source—a suspect can, therefore, be absolutely excluded (ignoring the possibility of human error or sample contamination).

Random match probability

This is derived from multiplying the probabilities for the individual loci (derived from frequencies observed in tested samples of specific populations) together—for 10 loci this is in the order of 1 in billions.


These are artificially generated small peaks which appear due to the chemistry of the PCR process—the problem with a ‘stutter’ is knowing whether it is indeed a ‘stutter’, or a small peak representing another source of DNA such as a mixed sample. This is a subjective decision made by the reporting scientist—a peak will usually be considered a stutter if <15% of the size of the main peak.

Mixed samples

These are those obtained from more than one source. In some cases, it might be straightforward to subtract a known source (e.g. the female in a case of alleged rape), but if DNA from multiple sexual partners is present, the result might be inconclusive. A number of markers on the Y chromosome have been identified. These can be valuable to discriminate between multiple male contributors in a mixed biological sample.

Partial profiles

These arise when it is not possible to analyse 10 loci—either due to the quantity or quality of DNA recovered from the sample. A random match probability can still be calculated, but this will have less discriminatory power and will carry less evidential value.

Interpretation of profiles

Conclusions and statements are made from interpretation of the profile as a whole, rather than a single locus, and this is particularly relevant when making conclusions as to the number of contributors to a sample. While two peaks indicates at least one contributor, if every locus of the profile had one or two peaks the profile would be taken as originating from a single source sample. The examples shown in Figs. 15.115.4 are illustrative of the possible results at a single locus.

                            Fig. 15.1 Peaks produced most likely by one
                                heterozygous individual. Examination of further loci might indicate
                                more than one contributor to the sample in which case a) at least
                                one heterozygous individual, or b) at least two homozygous
                                individuals, or c) a combination of homozygous and heterozygous
                                individuals who share one allele.

Fig. 15.1
Peaks produced most likely by one heterozygous individual. Examination of further loci might indicate more than one contributor to the sample in which case a) at least one heterozygous individual, or b) at least two homozygous individuals, or c) a combination of homozygous and heterozygous individuals who share one allele.

                            Fig. 15.2 Peaks produced by a) (at least)
                                two homozygous individuals—jnote that the smaller peak is
                                <50% of the larger peak and is unlikely therefore to have
                                come from a single heterozygous individual or b) one heterozygous
                                individual and one homozygous (for allele 19) individual.

Fig. 15.2
Peaks produced by a) (at least) two homozygous individuals—jnote that the smaller peak is <50% of the larger peak and is unlikely therefore to have come from a single heterozygous individual or b) one heterozygous individual and one homozygous (for allele 19) individual.

                            Fig. 15.3 Peaks produced by at least two
                                individuals (potentially a combination of heterozygous and
                                homozygous). This usually implies a mixed sample; however,
                                individuals with chromosomal abnormalities (e.g. Down syndrome with
                                trisomy 21) could produce a similar pattern depending on the loci

Fig. 15.3
Peaks produced by at least two individuals (potentially a combination of heterozygous and homozygous). This usually implies a mixed sample; however, individuals with chromosomal abnormalities (e.g. Down syndrome with trisomy 21) could produce a similar pattern depending on the loci tested.

                            Fig. 15.4 Peaks produced by either a) a
                                stutter and at least one homozygous individual, or b) a heterozygous
                                individual and a homozygous individual (for allele 14), or c) two
                                homozygous individuals.

Fig. 15.4
Peaks produced by either a) a stutter and at least one homozygous individual, or b) a heterozygous individual and a homozygous individual (for allele 14), or c) two homozygous individuals.

National DNA database

The UK database

The UK has the world's largest DNA database, with profiles from >5% of the population stored. In the USA, the comparable figure is <1%. Legislation governing samples obtained for and retained on the national DNA database is in a state of flux. The Crime and Security Bill 2009–2010 (England and Wales, Northern Ireland) proposes new time limits for the retention of DNA samples, DNA profiles, and fingerprints and extends the circumstances in which such samples can be collected. The Bill responds to a European Court of Human Rights judgement and represents a scaling-down of earlier proposals to enable indefinite storage of some profiles.


DNA databases are the subject of controversy, particularly regarding the potential uses of the data out of the criminal justice area (e.g. health-related information and insurance). Furthermore, as the number of profiles stored on the national DNA database increases, the likelihood of a coincidental or adventitious match also inevitably increases. The risk of a chance match would be significantly reduced if the number of loci tested was increased.

Alternatives to DNA analysis

Although DNA analysis has certainly taken over from a lot of other methods in becoming the technique of choice for forensic ‘identification’ purposes, it should be noted that other methods retain usefulness in certain circumstances:

  • Blood groups—a quick and easy method of elimination.

  • Anthropology—able to narrow a search on the basis of age and sex.

  • Fingerprints—may identify an individual not on a DNA database.

  • Cheaper techniques are likely to retain a role within the developing world.

DNA analysis in perspective

It must be remembered that without a reference sample, profiling a sample recovered from a crime scene will not result in a positive identification. DNA analysis is based on achieving a comparison between two samples, one of which has a known source. From the perspective of the criminal justice system, a national DNA database (with appropriate international communication) provides an important reference source—the civil liberty and ethical issues arising from calls to make it a universal database are issues still to be resolved.

Further reading

Gill P, Jeffreys AJ, and Werrett DJ (1985). Forensic applications of DNA ‘fingerprints’. Nature 318:577–9.

Exonerating the innocent

Almost invariably, techniques of forensic science and medicine are thought of as being used to convict the guilty. However, one of the earliest uses of DNA evidence was arguably even more important—to exonerate the innocent.

The Colin Pitchfork case

In 1983, Lynda Mann was raped and murdered in the Leicestershire town of Narborough. A semen sample identified a type A blood group and an enzyme profile matching 10% of the male population. In 1986, Dawn Ashworth was sexually assaulted and murdered in the same town—evidence from her body indicated the same attacker had committed both murders. Police had a suspect who confessed to the second killing, but denied involvement in Lynda Mann's death. Convinced the suspect was responsible for both deaths, the police approached Jeffreys who, with Gill and Werrett, had described the potential for using DNA profiling in forensic cases. Semen samples obtained from the two murder victims were compared against a suspect's blood sample. Analysis conclusively proved that both girls were killed by the same man. However, crucially, DNA evidence proved that this was not the suspect. Given that the police had a confession relating to the killing of one victim, it is probable that if the case had proceeded to trial, the suspect would have been convicted and the case closed. In addition to the miscarriage of justice that would have entailed, it would also have left the guilty man at liberty.

In order to identify the man responsible for the murders it was, therefore, simply a task of finding the individual who matched the two existing samples. Thus the world's first intelligence-led DNA screen took place involving 5000 men from three local villages who were asked to provide blood or saliva samples. Blood grouping was used to identify the 10% of men with the killers known blood group and DNA profiling was subsequently carried out on these individuals.

The guilty man almost escaped justice by getting a friend to give a blood sample using his name. Fortunately, a conversation relating to the switch was overheard and a local man, Colin Pitchfork, was arrested and later convicted. This demonstrates that even with the best scientific methods available human factors are just as important.

Sean Hodgson case

In 2009, Sean Hodgson was freed after spending 27 years in jail for a murder which he initially confessed to, but DNA techniques indicated that he did not commit. This period of imprisonment is one of the longest terms served following a miscarriage of justice in the UK.

Further reading

Gill P, Jeffreys AJ, and Werrett DJ (1985). Forensic applications of DNA ‘fingerprints’. Nature 318:577–9.

Forensic biology

The advent of DNA profiling has had a huge impact on the importance of biological evidence, making the latter seem less relevant, particularly in relation to the identification of individuals involved in a crime scene. However, the older techniques are still important in determining the type of biological fluid or sample that has been found, even if being less discriminatory for personal identification.

Blood identification

Blood is the body fluid most often deposited and most easily identified at a violent crime scene—it is red and visible to the naked eye. Indeed, the simple presence of blood can provide information about events at a scene due to its location and distribution on surfaces such as floors, walls, clothing, and moveable objects. Whether the blood is still wet or has discoloured with age, may provide some information regarding the time it was placed where it was found. However, blood is not always easy to find at a scene, so screening techniques should be used. It is noted that the presence of blood at a scene does not necessarily imply a crime—a sudden natural death from ruptured oesophageal varices, for example, may be often accompanied by copious quantities of blood.

Presumptive luminal test

A crime scene can be screened for the presence of blood by spraying the area with a solution of luminal and looking for and photographing the chemiluminescent glow produced by blood. The chemistry of the reaction involves an oxidization of the luminol compound by the haem component of red blood cells, including a release of pale blue or yellow-green light (depending on the reagent preparation). Luminol is very sensitive and thus, very useful, but it can give a false positive by reacting with other oxidizing chemicals.

Presumptive tests are useful in indicating the presence of a substance quickly and cheaply. They can be used at the scene. They provide qualitative, not quantitative, information and are not definitive. Other tests should be carried out at the laboratory. In practice, some scenes ‘obviously’ contain large amounts of blood, in which case such tests are unlikely to be carried out.

Stain collection and transportation

The way a stain is collected, packaged, and transported will depend on its location and whether it is wet or dry. If the stain is on a moveable object and has dried, it is possible to transport the whole stain, by putting the stained article (or sample if the article is too bulky) in a paper bag or envelope. Representative unstained areas of the stained item should also be collected and packaged separately from the stained samples, to act as a negative control. In some cases, a dried stain may be swabbed with a moistened cotton swab and transported on the swab in its protective container placed in paper packaging. The containers may be ventilated to assist drying of the swab.

Damp environments

Prolonged exposure to damp environments accelerates decomposition and growth of micro-organisms, which can compromise the sample, so samples should be dried as soon as possible. A scraping of dried material may be collected and transported in paper packaging, rather than swabbing the sample.

Wet articles which cannot be dried on site should be transported in plastic bags and dried out at the laboratory for subsequent storage in paper.

Further tests

Further tests carried out at the laboratory on the collected samples include chemical reactions that produce a colour change in the presence of the oxidizing haem component of red blood cells. Two commonly used tests are the KM (Kastle–Meyer) and LMG (leucomalachite green) tests.

Source of blood

The advent of DNA profiling technology has largely superseded the previously used identifying tests, which fell in to the discipline of serology.

Species identification

This is particularly relevant in the area of forensic wildlife or animal cases (e.g. illegal importation). Tests involve immunological components that give a visible precipitation stain when blood of a particular species is present. The precipitate results when antibodies, which are immunological molecules that have specific recognition characteristics such that they only bind other molecules of a particular shape and structure (which are species specific), encounter such molecules. The molecules to which antibodies bind are known as antigens and when the species-specific sets of antibodies and antigens that are used in the tests interact, a precipitate of the bound antibody–antigen complexes forms enabling the species of a sample to be identified.

ABO typing

Prior to DNA analysis, the science of immunology provided the methods to distinguish individuals on the basis of the presence or absence of specific antigens. The most common tests have been to determine the ABO blood group and phosphoglucomutase (PGM) type of a sample. Karl Landsteiner was awarded the Nobel Prize for Medicine in 1930 for his classification of the ABO blood system and determining compatibility/incompatibility for blood transfusions on the basis of this system.

ABO typing is useful and well established, although its power of discrimination is limited. There are three possible antigens of the ABO group which are present on red blood cells, namely A, B, and O and the combination of these results in four possible blood groups, A, B, AB, and O, which can be detected immunologically. Each individual inherits two of the antigenic variants, one from each parent, which may be the same or different (Table 15.1).

Table 15.1

Parent 1

Parent 2




















The frequencies of the different ABO blood groups vary from population to population.


When blood of an individual who is of the A blood group is mixed with antisera containing anti-A antibodies, the red blood cells and the antibodies will agglutinate, clumping together to form a precipitate. If antisera containing anti-B antibodies is added to red blood cells from an individual who is of the A blood group there will be no antigen–antibody reaction and the red blood cells will not precipitate. Red blood cells from an individual who is of the AB blood group will agglutinate with both anti-A and anti-B anti sera.

While it is possible to exclude an individual as being the source of an unknown sample if the blood types are not the same, one can only conclude that an individual who does have the same blood type as a sample could be included in the population of possible donors, along with others who also share the blood group type in question.

ABO and paternity testing

Blood group typing has also been useful in paternity dispute cases. An individual's blood group is a combination of the variants they inherit from their parents; each parent donating one variant to the child. If the blood group of a child could not result from the possible combinations of the blood groups of the known parent and the parent in dispute, then the parent in dispute would be excluded as the parent.

Phosphoglucomutase (PGM)

PGM is an enzyme involved in the metabolism of carbohydrate. PGM typing has a greater power of discrimination than ABO typing because there are 10 different forms that can be detected. PGM is especially useful because the enzyme can be detected in body fluids other than blood, for example, semen, saliva, and vaginal fluid.

One method of identifying the polymorphic variants uses a technique called isoelectric focusing (IEF). The four different PGM variants are identified as 2+, 2–, 1+, and 1– and each individual will have two variants, making up their genotype. The two variants making up an individuals genotype may be the same or they may be different, giving 10 possible combinations that can be differentiated by IEF.

While the prevalence of PGM in a number of body fluids is widespread, the finding of ABO type in non-blood body fluids depends on the secretor status of a person. 80% of humans are secretors and are a source of ABO-typeable material other than blood.

Blood pattern analysis

Whilst blood analysis may provide evidence of the source of a sample, it may also reveal important information as to the events that occurred at a scene of crime. The pattern of blood spatters, distribution, apparent age, and amount can help determine the sequence of events, including, for example, relative and absolute motion, position of the victim/assailant, and a time frame. Blood is a viscous fluid (about 6x as viscous as water) with a predictable behaviour. To form a blood spatter, the surface tension of the blood must be overcome.

Role of blood pattern evidence

Blood pattern evidence can be used to establish:

  • Direction of travel of the blood droplets and source.

  • Distance from source to impact surface.

  • Angle of impact.

  • A sequence of events (incident reconstruction).

It is important to understand the limitations of this type of analysis. Patterns are formed as a result of the relative motions between the source and the impact plane—these may not be easily or directly translated into absolute motions or pathways.

Blood drops

A blood drop which impacts at an angle of 90° will usually form a circular shape. The extent of satellite drops gives an indication of the velocity of impact (or if vertical, the distance fallen). A smooth non-porous surface such as a bathroom tile or glass is likely to produce less satellites (i.e. a smoother circle) than a rough surface such as a piece of wood. Arterial pressure and/or movement means that blood can be transferred in an upwards direction in addition to falling as a result of gravity.

The length (L) and width (W) of the drop are measured. The approximate angle of impact (Ai) is given by:

L / W = sin ( A i )

Distribution of blood

The largest quantities of blood are often located where the assault ended or, if the victim moved, at the place where they collapsed/died. This is because at the start of an attack, the victim is more likely to be mobile and blood will be distributed over a wider area. Furthermore, a continued attack will exacerbate existing and create new wounds. As the victim (and/or the assailant) become covered in blood, it is more likely that smears and staining will be present on walls and floors. If a body has been moved from the primary scene where an attack took place to a secondary site then blood may not be found: the absence of blood (in the presence of significant wounding) is an indication that a body has been moved.

Cast-off blood

Cast-off blood may be produced as a result of the centrifugal forces when weapons such as blunt instruments or sharp knives are swung in an arc. The blood marks along the arc will change shape reflecting the angle of impact. Cast-off should be distinguished from arterial spurt.


A wipe occurs when a moving object comes into contact with a blood stain. The direction of motion is usually apparent. A swipe is created when a moving object covered in blood comes into contact with a surface. The direction of travel is less obvious in this case, although thinning or feathering of the stain means that the mark gradually disappears in the direction of travel. Occasionally, especially when the initial contact was gradual and light, feathering appears on both sides of a swipe in which case the direction of motion is very difficult to ascertain.

Flow patterns

Since blood is a fluid, it moves under the influence of gravity. A change of direction in an otherwise uninterrupted flow signals movement. This can be observed if a body is moved soon after death (i.e. whilst blood is still liquid and able to move under gravity).

Fingerprints in blood

Fingerprints (in addition to more obvious patterns such as footwear) may be found in blood—these can be significant as they establish that contact occurred after the blood was deposited. Insofar as footprints represent the direction of travel, they can be useful when reconstructing the incident.

Origin of blood

Determining the point of origin of a series of blood marks requires convergence analysis. By estimating the parabolic ‘flight path’ of a number of points the origin can be plotted. Lasers and computerized reconstructions have replaced the traditional string pathways. Convergence analysis invariably requires the assumption of a single stationary point of origin; however, this is not always the case in practice.

In practice, it is rare to find patterns not obfuscated by other stains—these complex scenes require careful interpretation.

Forensic identification of semen


The identification of semen may be of relevance in a sexually motivated crime or allegation of rape. The presence of semen is not required to prove the crime of rape and the presence of semen at a scene does not prove that rape occurred.


Semen is not as visibly obvious to the naked eye as blood, although it will fluoresce when UV light is shone on it. However, urine will also fluoresce under UV light, hence, this is a presumptive test. An amount large enough to be seen by the naked eye will be a yellowish-white stain, but swabs will be taken for other presumptive and/or confirmatory tests to be done in the laboratory. Semen is comprised of sperm cells in seminal fluid. The most common presumptive test is for a constituent of seminal fluid called acid phosphatase (AP). The AP test involves the application of a cocktail of chemicals that will generate a purple colour when AP is present.


A commonly used confirmatory test is the microscopic observation of sperm cells. A smear is made on a microscope slide and stained with one of a selection of dyes (haematoxylin or Christmas tree stain) and then identified under a microscope (purple or red, respectively). The process is time consuming, especially the fewer the sperm cells there are and can be hard to interpret, depending on the amount of contaminating cellular material (e.g. vaginal or faecal matter). The number of sperm cells found is scored on an arbitrary scale of negative or 1–5 levels of positive. The presence of tails on the sperm cells is also noted as this is a certain identification for sperm; the problem being that the tails will degrade or separate from the sperm head very easily after ejaculation and depending on the conditions in which the semen is left or stored.

Prostate specific antigen

The assay for prostate specific antigen or p30 (prostate specific antigen, PSA) is an alternative confirmatory test that stains for another constituent of seminal fluid. It is a time-consuming immunological assay, but is proof positive for semen. It is especially useful in identification of semen with no or few sperm cells.

Location of semen

In suspected rape cases, the location of the semen within the body following vaginal or anal sex is very useful in determining whether penetration may have occurred at all and how long previously. Identification of an individual has been assisted by use of tissue types such as ABO group if secretor positive, or PGM testing.

Forensic identification of other fluids


Urine is generally not tested for in the laboratory as there is no good, specific assay. Being mainly a solution of salts in water with few cells, it is also not an optimal source for DNA testing. Its main use is in toxicological assays for alcohol and the presence of drugs.


Saliva is almost entirely water, but contains other substances, the most important of which is amylase, an enzyme used to digest carbohydrates.

Presumptive test

The suspect material is mixed with a starch solution. Iodine is added and this turns the solution blue-black if the starch is still intact. If amylase is present, it digests some or all of the starch and the test solution will stay yellow-brown (colour of iodine). This procedure for this presumptive test is not optimal and has been modified into the Phadebas test.

Phadebas test

Starch molecules are attached in an insoluble dye complex and when digested by amylase, it releases dye that becomes visible and can be measured or photographed, depending on whether the assay is done in a test tube or using Phadebas-impregnated paper. Although the chemistry is not very sensitive and other body fluids (e.g. semen, vomit, vaginal secretions) contain amylase giving a positive reaction, the relative concentration of amylase in saliva means that it gives a stronger reaction than other fluids. Amylase is relatively stable so positive results can be found after many months. As with semen, identifying an individual has been assisted by use of tissue types (e.g. ABO group, if secretor positive).


This is not a good source of evidence or identification, other than giving an idea of food intake. Faeces are mainly bacteria, undigested food residue, gut-lining cells, and blood pigment breakdown products. DNA profiling is inefficient because although many gut-lining cells are shed into the gut, these are usually dead and undergo bacterial degradation.


In addition to the anthropological information such as age, stature, and sex, carefully prepared bone samples can produce DNA for profiling. Best results are obtained with mitochondrial DNA profiling rather than standard profilin—much depends on the bone used and its condition.


Hairs and other fibres (whether natural or man-made) are usually considered trace evidence. Hairs and fibres are compared to reference samples using morphology and colour comparison. Plucked scalp hairs are likely to be healthy, alive, and intact, with a root of skin cells from the point of insertion in the scalp. These cells are sufficient to act as a source of DNA for profiling and can therefore be of evidential significance.

Forensic anthropology

The primary role of the forensic anthropologist is to identify and interpret human remains. In this task, there may be overlap with the forensic pathologist: neither should exclude the expert contribution of the other.


In addition to cases in which an unidentified human body is discovered, either by accident or as the result of a deliberate search, the discovery of skeletal remains is relatively common during building work and other excavations. Although the final and definitive identification of human remains may require DNA evidence, the forensic anthropologist provides a quick, inexpensive, and convenient way of narrowing down the focus of an investigation. Note that unless and until there is a reasonably small number of ‘suspects’, reference DNA samples are unlikely to be available. The preliminary questions to be addressed by an anthropologist should include:

  • Are the remains human or non-human?

  • Are the remains ‘forensic’ or archaeological?

  • How many individuals are there?

All skeletal material should always be referred to a forensic anthropologist or forensic pathologist for advice on these questions. Erroneous assumptions made by an inexperienced investigator at this stage are likely to be prejudicial to a successful investigation in the longer term.

If the answers to the earlier listed questions indicate that the remains are human and are of an age to be of interest to the medicolegal authorities (usually less than 50–75 years), the key tasks are to determine the biological profile:

  • Sex (sexual dimorphism, particularly the skull and pelvis).

  • Age (development and epiphyseal fusion).

  • Stature (derived from published formulae using the length of the long bones).

  • Race (skull morphology—although controversial).

The techniques employed to determine these primary characteristics vary according to the condition and extent of the remains available, the likely age range (for example, epiphyseal fusion has limited application in the mature skeleton), and the context of the discovery.

It is important not to quote precise findings for the primary characteristics. Age and stature should always be quoted as ranges and sex as a probability. There is considerable debate as to whether racial origins can be usefully determined by an examination of remains and findings should be reported with caution. Remember that the purpose of this examination is not to provide an individual identification, but to focus and guide the investigation which will lead to a positive match (most obviously by reducing the number of potential matches from a missing persons list).


Skeletal evidence of disease can reveal information about the deceased's medical history. Unless these conditions had been diagnosed during life, this is unlikely to provide any assistance in making an identification.

Disarticulated bones

The forensic anthropologist is able to interpret disarticulated bones. This may provide crucial evidence of animal disruption and ante- and/or postmortem dismemberment (including cannibalism). This evidence is likely to be critically compromised if the remains are not retrieved, stored, and transported by an experienced forensic anthropologist/archaeologist.


Evidence of trauma either prior to or as the cause of death can sometimes be recognized on skeletal remains. In mass grave excavations, bodies frequently reveal evidence of gunshot injuries—the site and characteristics of the bony injury can be useful to distinguish between, for example, execution (which may be illegal) and injuries received during battle (which may not).

Mass disasters

Although in many major incidents, bodies are recovered almost immediately, this is not always the case. If remains are severely decomposed, mutilated, or otherwise unrecognizable, a forensic anthropologist can provide useful clues as to identity. These should be confirmed, if possible, by DNA techniques prior to repatriation and appropriate disposal, as mistakes in identification can be devastating to families and relatives (as well as to the reputation of forensic services and international agencies).

Human rights abuses

Particularly in conflict and ex-conflict countries, the forensic anthropologist investigates allegations of human rights abuses and war crimes and provides evidence for criminal proceedings (usually under the auspices of the United Nations). The work frequently involves the excavation and interpretation of bodies from mass graves and can identify injury types, use of child soldiers, postmortem mutilation, and provide evidence to identify and subsequently repatriate remains.

Body farms

Much information about the process of human decomposition has been gained from research performed at ‘body farms’ (see Forensic science [link]).

Further reading

British Association for Human Identification: Forensic science

Forensic Anthropology Society of Europe (FASE): Forensic science

Forensic archaeology

The primary task of the forensic archaeologist is to locate and/or retrieve concealed human remains and to do so in such a way that evidence is preserved. Whereas the activity of the forensic anthropologist takes place in the mortuary or laboratory, the forensic archaeologist works at the scene. In the USA, the terminology is ambiguous and the role of the archaeologist tends to be included under the term anthropologist.

It should be emphasized that forensic archaeologists do not search for ‘dead bodies’. Rather they search for various anomalies, such as soil disturbances which are indicators that a body (or drugs, illegal weapons, money, etc.) may be buried. Trained dogs are also available to locate bodies (similar to the use of dogs in avalanche rescue situations). The majority of concealed (as opposed to buried) bodies are discovered by accident in which case the expert assists with the recovery and preservation of evidence from what is likely to be a disturbed and contaminated scene. A body buried by a perpetrator can remain undiscovered despite extensive searches (e.g. the body of one of Ian Brady's victims, Keith Bennett, has never been located on Saddleworth Moor), although other apparently unrelated events might provide the vital preliminary clue (e.g. following Peter Tobin's conviction for the rape and murder of Angelika Kluk in 2007, police searched a house where he had previously lived and discovered the body of Vicky Hamilton who had disappeared in 1991).

Tools for searching include:

  • Aerial photography (preferably at dawn or dusk as angled sunlight casts shadows and highlights 3D features).

  • Magnetometry (variations in magnetic fields).

  • Ground penetrating radar (2D profiles of reflected radar pulses are plotted to generate a 3D image of the surveyed area).

  • Resistivity (near-surface variations in electrical resistance).

A forensic botanist (or a botanist with the experience of acting on the instructions of a legal authority) will be able to interpret plant repopulation following soil disturbance.

The decision to use line-searching/walking should be a balance between using small numbers of trained experts and (often significantly) larger numbers of willing and enthusiastic volunteers. If volunteers are forthcoming, then searching is optimized by interspersing trained experts throughout the line and focusing expertise on areas identified as being likely locations. Those in charge of coordinating such a search should never forget that community resources might be extremely useful when appropriately managed, yet can often feel excluded if offers of assistance are dismissed without adequate explanation.

Excavation and retrieval

During excavation, which may involve horizontal or vertical trenches, the trained archaeologist must document and record all findings, including taking note of position (absolute and relative) using GPS (global positioning system) when appropriate. Subject to the burial matrix, it might be possible to recover toolmarks, footprints, and other impression evidence in addition to physical items such as bone. Particular care must be taken to identify and recover all the small bones (such as the phalanges). The absence of these bones at a burial site might be evidence that the body was moved from a previous location. The absence of these bones back at the mortuary should never be the result of inadequate excavation or recovery.

Examination and interpretation (either by the same individual or a forensic anthropologist and/or forensic pathologist) should, if possible, take place at a well-lit, fully serviced mortuary.

Forensic entomology


The association between flies and death (or dead bodies) has been known for a long time. The Ancient Egyptians appear to have understood the connection between the dead body, maggots and flies (Book of the Dead, Chapter 154), and Homer refers to swarms of flies around bodies on the battlefield (The Iliad XIX). The earliest reported use of insects to solve crime appears in the works of Sung Tz'u (1186–1249) who used the fact that flies swarmed to a murder weapon (a sickle) to elicit a confession from the guilty man. In the 16th century, the Vanitas school of Dutch artists often depicted the evanescent nature of human desire by placing a fly on a skull, whilst Emily Dickinson's (1830–1886) poem Dying focuses on a single fly in a room.

Body farms

There are several ‘body farms’ in the USA which are involved in the study of forensic entomology (see Forensic science [link]).

Determining the postmortem interval

The most frequent role of the forensic entomologist is to estimate a postmortem interval. However, it is important to be aware that it is only possible to quote a time that the body has been exposed to oviposition (egg laying). This is not necessarily the same as the time since death. This can be significantly different if, for example, the body has been moved or conditions have changed significantly in the interim.

There are two different methods of determining the time since oviposition, both based on the fact that insects are poikilothermic (their development rate fluctuates with temperature). This growth-rate/temperature relationship has been established in the laboratory for various species. It is essential to establish the identity of the species: this may be possible from larvae if they are at an advanced stage of development, otherwise rearing to adulthood (in the laboratory) is necessary. DNA profiling is likely to become useful in this area. Correct identification is essential, as different species have different developmental rates.

The comparative method compares the oldest specimen recovered from the crime scene to a development rate chart which gives the number of development hours (i.e. the time taken for the species to grow to the observed state of development) for various known temperatures. The temperature at the crime scene usually has to be estimated (even if the temperature is accurately measured after discovery, this only approximates to the temperature fluctuation during the period from the crime to the arrival of the law enforcement officers).

If the temperature at the scene (or, more likely, a nearby meteorological station) is known more precisely then the linear relationship between developmental rate and temperature can give a more accurate time since oviposition. A given species takes the same number of temperature units (known as accumulated degree hours, ADH) to develop to the same stage (within the upper and lower threshold limits for that species). For example, for Calliphora vicina (bluebottle blowfly), the development from egg to adult requires approximately 7200 ADH. It is possible, therefore, to sum the number of ADH and back-count on the development rate chart appropriate to the identified stage of larval development at the scene. An alternative to the ADH method is to use accumulated degree days (ADD), although if the temperature can be determined on an hour-by-hour basis, the ADH is more precise.

It is important to be aware that several variables affect the result and the postmortem interval should be regarded as an estimate, not an exact time. Variables include: location (the minimum developmental temperature varies significantly according to location), humidity, species mix, microclimate, maggot masses (which affect temperature), movement, and method of concealing the body.

The entomologist should attend at the scene if possible. Otherwise, specimens should be collected from the scene (flying insects—using an insect net) and body. Larvae should be collected according to a protocol agreed with the entomologist.

Other uses of entomology

These include identification of individual hosts by blood-meals (DNA analysis of haematophages), neglect (elderly or the young based on insects attracted to faeces), drug analysis (drugs can be detected in larvae feeding on a body), transportation, and smuggling (localized species found in shipments).

Accident investigation

Stings from bees and wasps can be a causal factor in accidents. Reactions to stings are classified into three degrees of severity:1

  • Hymenopterism vulgaris—the majority of cases. Painful, but not serious or lethal although accidents can result from loss of concentration or panic.

  • Hymenopterism intermedia—usually non-lethal, but caution is required when swelling of the tongue, neck, or throat results in impairment of swallowing and breathing.

  • Hymenopterism ultima—lethal or near- lethal reactions to stings, usually as a result of anaphylactic shock.

Insects are a potentially significant threat as a result of bioterrorism, as they may be used as vectors to carry disease or other hazards. As weapons for military use, their significance depends on the extent to which their behaviour can be modified to achieve a particular objective.


1 Fluno JA (1961). Wasps as enemies of man. Bull Ent Soc Amer 7:117–19.

Environmental forensics

Definition and scope

The techniques of forensic investigation have extensive application to the legal implications of human activity on the environment. The term was first used after the Exxon Valdez oil spill in Alaska (1989). The remit of ‘environmental forensics’ is wide-ranging, overlaps with other disciplines, and includes:

  • Pollution (land, sea, noise).

  • Chemical contamination (and decontamination procedures).

  • Waste disposal and management.

  • Wildlife crime (live animal smuggling, trade in animal products, endangered species).

  • ‘Conflict diamonds.’

  • Fire behaviour in rural environments (particularly relevant to the investigation of bush and forest fires).


Techniques include those derived from a number of different disciplines, including analytical chemistry, microbiology, biochemistry, geosciences, hydrogeology, and atmospheric physics, in addition to those routinely used by forensic scientists in other areas. Given the ever-increasing amount and scope of legislation relating to the environment, it is important that breaches are thoroughly investigated and that an evidence-base is established on which to prosecute offenders (often on the basis of corporate and strict liability).


If law-enforcement officers suspect activity relating to any of these disciplines, it is imperative that advice from an appropriate expert or agency is sought as soon as possible.

Further reading

Convention on International Trade in Endangered Species of Wild Fauna and Flora: Forensic science

Environment Agency: Forensic science

European Environment Agency: Forensic science

Partnership for Action against Wildlife Crime Forensic science

Scottish Environment Protection Agency: Forensic science

United States Environmental Protection Agency: Forensic science

Document analysis

The field of document analysis is wide-ranging and includes various scenarios in which questions are asked: forged handwriting and/or signatures (e.g. cheques, wills), identifying the author of handwritten material (e.g. blackmail letters), identity of printers, photocopiers, and other devices, timescale of entries (e.g. diaries, accounts), origin of paper, and tracking amendments and alterations.

Handwriting analysis

This includes authentication of a wide range of handwritten documents. Graphologists claim that handwriting reflects personal character traits—this is a different area entirely (and one with little forensic application). Letters are examined and compared on the basis of their shape and their construction (stroke order and direction) and also their relation to adjacent characters. Ideally, the comparison should be between the same text written in the same format—either capital letters (upper case), script (letters are not joined), or cursive (joined-up writing without lifting the writing instrument). This sample might be available from other handwritten documents or can be requested from a suspect. The latter approach has the advantage of being able to control the pen, paper, and text, but it also tends to emphasize variations resulting from stress and of course, introduces the risk of deliberate disguise. An expert opinion can state that the comparison is conclusive, supportive, or inconclusive as to whether the compared documents were written by the same or different individuals. Note that the assumption that an individual's handwriting is unique has little or no scientific or statistical basis.


Although signatures are a form of handwriting, by their very nature they are highly specific. Forged signatures may be traced (so they lack natural flow) or practised (so they often lack accuracy). It is not possible to state that a signature is forged simply because it is dissimilar to another example, because of natural variation. Indeed, signatures which are too ‘identical’ should raise suspicion. When examined under a low power microscope, a forged signature may exhibit pauses and re-starts of strokes, jerky strokes (by writing very slowly) and evidence of tracing (pencil marks, indentations). These observations are objective. More subjectively, the expert may comment on morphological discrepancies (especially the proportions of characters) and such things as the position of the signature with respect to other content of a document.

Other document analysis

In addition to handwritten documents, inks, printing methods (e.g. laser, inkjet, dot-matrix), paper and other characteristics of printed documents can be analysed to establish date, source and potential falsification.

Further reading

Chip and pin card security: Forensic science

Security features of Euro banknotes: Forensic science

Forensic miscellany

As indicated in the introduction to this chapter, any scientific discipline which finds itself involved in a case of legal significance can be regarded as a ‘forensic science’. Some of those not previously discussed include branches of engineering, soil analysis, palynology (pollen analysis), ear identification (the European FEARID project), dendrochronology (used, famously, in the Lindbergh kidnapping case and also in several art and musical instrument fraud cases) and limnology (including the use of diatoms to diagnose freshwater drowning). All of these have some aspects which are considered controversial, but there are many examples of their use in successful prosecutions.

Soil analysis

The analysis of soil is particularly complex as it comprises inorganic and organic substances. Inorganic parts include particles of stones, sand, as well as chemicals such as salts. Organic constituents include dead and decaying vegetal matter, as well as pollen, microscopic insects, worms, and bacteria. Some attempts are being made to create databases of soil composition from different geographic regions and land uses. Potentially, the DNA profile of entire bacterial mixtures found in soil samples can be generated for identification purposes.


Different plants produce pollen of different sizes, shapes, and quantities at different times of the year. Microscopic examination of clothing and other material or swabs of body surfaces, will often detect complex mixtures, and sometimes very specific types, of pollen. This may be used to identify the geographical source or location of origin of the pollen which may be useful in tracking the whereabouts of evidential material. The difficulty may not be so much in the ability to identify the pollen, but to evaluate its meaning under the circumstances.

Forensic accountancy

Although not covered by the usual meaning of the term ‘science’, it is worth also mentioning disciplines such as forensic accountancy. Crimes such as money laundering, fraud, and Internet or computer-related offences, often require extremely detailed analysis of financial transactions and electronic transfers of funds (almost invariably with no physical ‘paper trail’).

Other activities

Of greater controversy are those activities with little or no published or verified scientific basis, but are promoted by individuals as aids to investigation. These include various para-psychological or pseudo-scientific ‘techniques’ often promoted by charismatic individuals who target families or relatives of victims (whilst also claiming success with law enforcement agencies). It should be noted that if an individual ‘predicts’ the location of, for example, a murder victim often enough then they will, inevitably, get it correct eventually—this may well form the basis of the ‘success’ story. Whether or not such activities can assist an investigation is open to argument: what is less equivocal is the fact that they do not produce evidence which is likely to be accepted by a court of law (they would almost certainly fail the Daubert standard for admissibility of scientific evidence—see Forensic science [link]).

Note that there are various scientific disciplines, such as forensic psychology, which have an important role in informing an investigation, although they may provide little or no evidence which can aid conviction. A psychologist may, for example, interpret an incident to state that this-or-that scenario is more-or-less likely (often based on documented series of apparently analogous cases). This can often significantly benefit an investigation by providing an initial focus so that resources can be targeted appropriately. However, to suggest that an individual is, therefore ‘guilty’ on the basis of this type of analysis would be to commit a fallacy of induction.

Fire: background

Conditions for fire

Four conditions are required for a fire to ignite and continue to burn:

  • Combustible material (fuel)

  • Oxidizing agent (usually O2)

  • Energy (usually heat)

  • A self-sustaining reaction.

Fuel and O2 must be present in suitable proportions for combustion to occur. There are defined upper and lower limits out of which a fuel will not ignite. These are termed the ‘flammability limits’ of the fuel.

Energy may be generated by mechanical means, for example, friction as in the striking of a match; electrical means, for example, arcing or short-circuiting of electrical appliances; gaseous compression; and chemical or nuclear reactions.

Continuation of a fire

Once a fire starts, the heat produced from the exothermic reaction is usually sufficient for continuous re-ignition to occur and provided a supply of fuel and O2 is maintained, the fire will continue. Removing one or more of these requirements, however, will result in the fire being extinguished—this provides the theoretical basis for fire-fighting.

Types of fire

Fires may be distinguished as flammable combustion or smouldering combustion, depending on whether flames are present or not.

Flammable combustion

Flames result from the burning of flammable gas. This gas may be the fuel itself causing the fire (e.g. natural gas); it may result from the pyrolysis (chemical breakdown due to heat) of solid fuels generating flammable gases; or from the vaporization of flammable gases from liquid fuels, such as petrol.

Smouldering combustion

In contrast, smouldering combustion, which is seen at the interface between solid fuels and air, is devoid of flames, generating only heat and light. Solid fuels, such as wood, which are based on carbon compounds, which pyrolyse to produce flammable gases, also produce char (impure carbon remains) and this char may continue to smoulder after the pyrolysis products have burnt (the basis of a charcoal barbecue).

Fire prevention and safety


In the UK, various regulations (e.g. The Furniture and Furnishings (Fire) (Safety) Regulations 1988 as amended) govern the ignition characteristics of materials used in furniture manufacture. These were introduced in response to concern about the number of fatalities arising from fires involving polyurethane foam. The regulations are out of the scope of this book, but they are relevant to investigators seeking to determine the cause of a fire and the cause of death in a fire-related fatality. Furniture made prior to 1950 is exempt from the regulations, but furniture of this era is still in widespread use. All furniture and furnishings to which the regulations apply should be appropriately labelled. Furniture donated free of charge (e.g. by a charity) or given away by an individual is not covered by the regulations. In most cases, landlords who rent homes as part of a business are obliged to ensure that any included furniture adheres to the regulations. The regulations in the UK are considered to be stricter than those in the EU or USA.

Toxicological analysis

Although CO is the most frequent killer in fatal domestic fires, the toxicologist should analyse postmortem samples for the highly toxic products of foam combustion (e.g. HCN) as this provides an audit of the effectiveness of this type of legislation.

Smoke detectors

The most significant fire safety improvement in the domestic setting has been the widespread installation of smoke detectors/alarms. These are required by building regulations to be fitted into new houses. Landlords letting property on a commercial basis must ensure adequate (as determined by the size of the house, for example) smoke alarms are fitted (including mains-powered alarms). Multiple occupancy properties (houses in multiple occupation, HMO) and commercial properties are covered by specific legislation which includes the use of fire doors and protected escape routes. Smoke detectors/alarms and fire doors are designed to increase the chance of escape once a fire has started: the regulations referred to here are intended to decrease the risk of fire starting.

Water sprinklers

Water sprinkler systems are relatively rare in domestic properties, but legislation requires the fitting of a sprinkler system in larger commercial properties. The Theatre Royal, Drury Lane, London, was fitted with a sprinkler system in 1812.

Behaviour of fire indoors

Observations of test fires in controlled laboratory environments, using defined settings and parameters, provide information regarding the course of a fire. Real fires may behave unpredictably because of unknown variables. Fires that burn indoors in a room can be described as going through a sequence of stages, namely:

  • Ignition

  • Growth

  • Flashover

  • Fully developed fire (post-flashover steady-state burning)

  • Decay.


A fire will ignite only if four conditions for ignition (see Forensic science [link]) are present. Flame may be immediate or only after a smouldering fire has generated sufficient heat. A fire may smoulder for a period before self-extinguishing.


Fire may spread from the point of ignition (the ‘seat’). The spread will depend on the availability of O2 and position of fuel relative to the seat. Modern fabrics may contain substances that actively retard the fire development. Heat required for growth may be transferred by one of three mechanisms: thermal convection, thermal conduction, and radiation.

Thermal convection

Transmission of heat by the movement of gas or liquid molecules away from the source of heat. The flow of heated molecules set up by this mechanism creates convection currents and these currents of hot gas rise upwards from a fire due to the increased buoyancy of the heated gas molecules. In this way, a fire that starts at a lower level in a room will produce pyrolysis products that rise to form a layer of hot gas at the top of the room, below which the temperature of the air in the room will remain relatively cool. Convection is usually the most efficient mechanism of transferring heat from burning fuel and because of the upward currents, a fire in a solid framework will tend to burn upwards more readily than downwards or outwards.

Thermal conduction

This is the movement of heat through a medium as energy is transferred from molecule to molecule, without there being macroscopic movement of the medium (e.g. transfer of heat along a metal rod). The contribution that conduction plays in spreading heat in a room fire will depend on the thermal conductivity of the components and furnishings in the room.


This is the transmission of heat in the form of electromagnetic radiation (largely responsible for causing flashover). Fuel above a flame may catch light and start to burn, which allows the fire to spread upwards. The extent of the flames depends on the availability of fuel and O2.

A fire will usually burn in a conical shape, the narrowest part being at the bottom spreading outwards towards the upper regions of the affected area. This is provided the fire can spread outwards in all directions. If the fire is curtailed by a vertical surface (e.g. a wall), then a burn pattern in a shape approximating to a V will be formed on that surface.


This is the ignition of the hot pyrolysis products that have risen to the ceiling by convection. It results in a rapid engulfment of the ceiling in flames and increased temperature of the affected area.


This occurs when items above the level of the fire, for example, curtains or larger structural components, fall to the ground and ignite. If dropdown is available in significant quantities, then this maintains a supply of fresh fuel to the fire. Ignited dropdown should not be confused with multiple fire seats (which have a high suspicion of being caused deliberately).


This describes the instantaneous ignition of all the fuel (gas, solid, and liquid) in the room and occurs if the temperature in the room increases sufficiently (often following flameover). Flashover signals the end of effective search and rescue as it involves extremely high temperatures (>500°C). It is the transition from a content fire to a structural fire. Delaying flashover can create valuable search and rescue time and can be achieved by allowing the escape of heat, limiting the supply of O2 and using fire fighting equipment to reduce the temperature in the room.

The investigator should have an understanding of flashover as it replicates the effects of, and can easily be mistaken for, an accelerated (arson) fire (e.g. wall charring from floor to ceiling). It also destroys more subtle evidence which might have pointed to a seat of ignition and evidence of criminality.

Fully developed fire (post-flashover steady state burning)

Following flashover, the heat release rate in the burning room will be at its maximum level as all of the available fuel in the room is involved. The rate at which the fuel burns is limited by the air supply: the fire is said to be ventilation controlled. It is likely that the rate at which pyrolysis products are produced will exceed the rate at which the O2 required to burn these gases is supplied. Partial combustion and pyrolysis products that have built up, and which are present in hot smoke leaving the burning room may result in flameover.


The limiting factors to an indoor fire are availability of fuel and O2. The source of energy (heat) is usually provided by the fire itself and is only limited if fire fighting occurs. If an indoor fire is limited by O2, sufficient levels of pyrolysis products may still be produced so that, if the room is suddenly ventilated, rapid flaming combustion may again be supported. This is termed ‘flashback’.

Behaviour of fire outdoors


Outdoor fires can be naturally occurring phenomena or result from human activity. Fires which are of human origin may be started with deliberate and malicious intent (‘arson’) or inadvertently. The latter includes those fires which were started with a specific purpose (e.g. barbecue, bonfire), but which later spread out of control.

Development and spread

Outdoor fires follow a less complex sequence of events than indoor fires. A fire in an open area of horizontal ground will generate convection currents as the hot gases rise from the seat of the fire. At the same time, air will flow towards the base of the fire, so O2 will not be in short supply, but whether the fire will spread will be dependent on fuel being adjacent to the base of the fire and the rate at which the fuel can be heated. Because of the flow of convection currents, hot gases will be carried up, so heating of the ground level fuel will be dependent on conduction and radiation. The fire will spread in a shape and course dependent on the characteristics of the surrounding fuel (type, size, distribution, and moisture), the weather, and environmental conditions (the topography). Fire will travel up-slope more rapidly than down-slope, as the flames preheat the fuel and make the process of ignition more rapid.

Fire barriers

Fire barriers are areas which contain no fuel. They are used to limit the spread of fire. If wind conditions are favourable, fire has the potential to ‘jump’ across these breaks. Thermal convection heats material across the barrier and particularly if the wind is sufficiently strong to transfer hot embers, fuel can ignite at remote locations. When used to protect property, fire barriers are known as ‘Asset Protection Zones’. They comprise part of the design of developments which lie in proximity to high-risk fire areas.

Fire: investigation

The investigation of a fire scene should follow the procedures applicable to more general crime scene investigation. Fires starting accidentally may have public safety and legal implications (tort/negligence, Health and Safety), whilst deliberate ignition can give rise to various criminal charges (including murder).

Preserving life or evidence?

Perhaps more than in any other scene, there is often an inevitable conflict between the task of preserving life (and property) and the preservation of evidence. Before any evidence gathering can be allowed to take place, the fire must be extinguished (itself a process which is likely to compromise evidence) and the scene must be declared safe by an appropriately qualified person.

It is crucial that all investigators work together as a team and adhere to a single and consistent line of command. It is usual for the Fire and Rescue Service to lead the investigation of a fire, although other specialists will be called upon when relevant (e.g. the forensic pathologist will investigate fatalities).

Origin of fire

The primary questions in any fire investigation are to determine the seat and the cause of the fire. The detection of accelerants is a major part in the determination that a fire has been started deliberately, although this information must always be interpreted in conjunction with the circumstances of the scene and other evidence (witness statements regarding spread, forced entry, etc.).

Multiple seats

The presence of multiple seats tends to indicate that a fire was started deliberately, but these must be distinguished from dropdown and other incidental findings.

Smell of vapours

The smell of vapours at a scene suggests that an accelerant has been used in a fire. Trained sniffer dogs are potentially useful aids to an investigation. In industrial settings, it is more likely that fire accelerants might have been present for other reasons, so it is important to ascertain what chemicals if any, were present before the fire started.

Collected samples

Samples which are collected should be stored and transported in nylon bags. These are suitable because the level of hydrocarbon leakage through the bags is low (though they are permeable to alcohol and water). Other materials (for example, polythene) allow leakage of the typical accelerants that may be used at a fire, resulting in loss of evidence and the potential for contamination with other samples. Contamination is a significant problem with fire scene samples.


Samples are typically analysed by GC–MS (gas chromatography–mass spectrometry) using samples from the head-space of the test sample. This process allows the components of the sample to be separated, giving a characteristic fingerprint of different fuels. The sensitivity of GC–MS allows trace as well as bulk samples to be analysed, and its ability to separate different components in mixtures enables the different components of mixtures to be isolated, from which identification may follow.

Interpretation of the analysis of samples taken from a fire scene can be difficult because of the confusion between the chemical profiles of samples and the other products of combustion or the breakdown products of hydrocarbons. Whilst fuels such as petrol and diesel have a characteristic fingerprint, components within these fuels and their breakdown products may correspond with those of furnishing and carpet pyrolysis products, making it different to tell if the analysis has detected the presence of an accelerant or the effect of fire from combustion of products normally present during a fire. Reconstruction of fire scenes is an important part of determining what happened to cause a fire and can aid interpretation of the results of the scientific analysis.


Arson is the deliberate and malicious setting on fire of property either owned by the individual (usually a fraudulent insurance claim) or by another person. Motives for arson are various and include revenge, fraud, vandalism, pyromania, ‘enjoyment’ (particularly the power and control over the emergency services), and the concealment of crime (including murder). Schools are frequently targeted by pupils/ex-pupils. Deliberate fire-setting is not limited to buildings, but can also involve waste ground and bush or forest areas.

Explosions and explosives


An explosion results when pressure in a space increases at a rate greater than can be dispersed or exceeds the ability of a containing vessel to hold it. There is a rapid release of energy, causing a violent effect. The explosive process can be mechanical, thermal, electrical, nuclear (fusion or fission), or as is the case in the majority of deliberate explosions, chemical.

Chemical process

The explosive process is initiated by supplying sufficient energy to the oxidizing agent component and the fuel component. A primary explosive may be used to initiate the secondary explosive. Firearms' cartridges work along these lines with the primer (primary explosive) triggering the propellant. The fuel component and oxidizing agent part of an explosive may be combined by physically mixing the components together or they may be part of the same compound—for example, nitroglycerine (glyceryl trinitrate) and TNT (2,4,6-trinitrotoluene). Inert substances may be added to make the explosive safer to handle (e.g. dynamite, which is based on nitroglycerine). Explosives which rely on physically mixing separate fuel and oxidizing agents together may use fuels such as sugars or other carbohydrates, carbon or hydrocarbons, powdered metals or sulphur, and common oxidizing agents including inorganic nitrates, chlorates, and percholates. Gunpowder is a mixture of carbon, sulphur, and potassium nitrate.

Detonating explosives

Otherwise known as ‘high explosives’, these do not usually need to be contained in order to explode. High explosives that are contained (e.g. military applications such as shells, grenades, bombs, mines), not only produce the powerful blast of the explosion, but also send fragments of the casing at high speed. In a detonating explosion, the reaction travels through the explosive medium at a speed in excessive of the speed of sound.

Low (deflagrating) explosives

In contrast to high explosives, these need to be contained in a vessel in order to explode. The direction of the subsequent blast can be controlled by having a deliberately constructed path of least resistance. The propellant of firearms is an example of a deflagrating explosive, as are flammable dusts such as flour, and mixtures of air and natural gas.

Condensed and dispersed explosives

Condensed explosives (solid or liquid) and dispersed explosives (gas or an aerosol, i.e. a fine dispersion of solid or liquid in a gas) may detonate or deflagrate, but detonations of dispersed explosives are rare.

Effects of the explosive blast

The destruction or effects of the explosive blast may indicate the type of explosive involved. In a condensed explosion, the maximum damage tends to coincide with the explosion centre (which may form a crater). The pressure of the detonation decreases away from the explosion centre. Detonation of a dispersed explosion, however, results in an increase of pressure away from the point of origin. There is a less clear relationship between pressure and distance from the explosion centre in deflagration of a dispersed explosive.

Detonations of condensed explosives cause maximal damage, often with a shattering or pulverizing effect. Deflagrations of condensed explosives tend to cause less blast damage, bending being the typical damage (often accompanied by extensive heat damage at the site of the explosive). In a dispersed deflagration, objects are often displaced, rather than shattered (walls may be pushed outwards) and objects within the actual explosion may be relatively unscathed. While craters may form when a condensed explosive is detonated on a surface, they are not produced by deflagration of a dispersed explosive. Explosions may also lead to fires.

Explosion investigation

The investigation of a scene of an explosion follows the principles as outlined for any scene of crime investigation. Whenever an explosive substance is suspected (either from observation of damage or from intelligence-led investigation) an appropriate explosives expert should be on scene. The possibility of similar unexploded devices (or ‘booby-traps’ if placed deliberately) should be assumed until the area has been declared safe. If fire starts as a result of an explosion, the priority is to extinguish this and ensure the scene is safe before proceeding.


The fact that casualties may need to be attended to close to the scene of the explosion is an additional complication. Officers in charge should prioritize the need to maintain order amongst the likely chaos and panic resulting from such events.

Scene control

The scene of an explosion can cover a considerable area. As far as is practicable, the cordon should include all this area. Note that as distance from the explosion centre increases, the fragments of evidence are likely to get smaller, so a search must be thorough and comprehensive. Items outside a cordon are liable to be collected by the public (including ‘souvenir hunters’) and important information may be lost (including information which may be helpful in identifying victims). Debris may include parts of the explosive device (detonator, timer, container if used, remote control, etc.) which need to be collected and examined. Such material may lead to laboratory identification of remains of an explosive, origin of the components, how the device was constructed, triggered, and potentially the individual(s) or organization involved. It is not uncommon for terrorist or criminal organizations to claim responsibility for large explosions and although certain authentication procedures may be in place, this should not replace thorough investigation.

Unintentional explosions

In addition to the obvious criminal implications of deliberate incidents, explosions (such as gas explosions), resulting from poor maintenance of infrastructure and/or illegal installation, whilst not being deliberate, may lead to prosecution under tort/negligence and/or Health and Safety legislation.


A number of techniques are used to analyse suspected chemical explosives. If a sample is suspected of containing a volatile explosive compound (e.g. nitroglycerine), then GC–MS may be used. Other chromatography methods, such as HPLC (high performance liquid chromatography), TLC (thin layer chromatography), or IC (ion chromatography, which separates ionic or polar species) are also used. SEM-EDX can identify which elements are present in a sample and IR spectroscopy can provide information regarding the chemical species or functional groups of the sample. Atomic absorption and emission spectroscopies may also be employed to analyse suspected explosives.

The context effect and scientific evidence

In forensic medicine and science, as in most areas of life, the context in which one looks at something changes how we perceive and interpret what we ‘see’. Optical illusions and ‘magic’ tricks work on this principle—the natural tendency to interpret what we ‘see’ according to what we expect, given previous experience and other contemporaneous information including what we are told.

In a forensic context, one key question is how much should an expert know about the case before they carry out their examination? This is a balance between having sufficient knowledge of the circumstances of the event in order to make the appropriate investigations, on one hand, and relying too heavily on assumptions and ‘context’ on the other. It needs to be remembered that a complainant does not necessarily admit to all aspects of the alleged offence immediately and the expert should never miss finding evidence simply because there was no reported reason to look. This is particularly important when examining a child.

In a case of a sudden death of a young known drug user, toxicological tests are likely to be instructed routinely. But what about the apparently natural death of an elderly person? One need look no further than the Shipman case (see Forensic science [link]) to see an example of how easy it is to accept an experts opinion simply because it seems to fit what we assume happened.

To admit bias in the scientific process is not to admit dishonesty or lack of integrity. It is simply to accept that individuals use direct experience and interactions in different ways. The good investigator will be aware of these and take them into account when analysing scientific results and evaluating and interpreting evidence.

Further reading

Dror I, Charlton D, Péron A (2006). Contextual information renders experts vulnerable to making erroneous identifications. Forensic Sci Int 156:74–8.

Saks MJ, Risinger DM, Rosenthal R, et al. (2003). Context effects in forensic science: A review and application of the science of science to crime laboratory practice in the United States. Sci Justice 43(2):77–90.

Forensic statistics

Why are statistics relevant to forensic medicine? Expressions such as ‘beyond reasonable doubt’ and more obviously, ‘on the balance of probabilities’ are probabilistic statements. When dealing with complex scientific evidence, experts should be fully aware of statistical implications. Dangers associated with statistical and probabilistic evidence are more than adequately illustrated by the paediatrician Professor Roy Meadow.

‘Meadow's law’

The aphorism, ‘one sudden infant death is a tragedy, two is suspicious and three is murder until proved otherwise’ became embodied in forensic orthodoxy during the latter part of the 20th century.1,2 Sally Clark was convicted of murder in 1999 and released on appeal in 2003. The evidence of Meadow was crucial to Clark's conviction (and also in the similar Anthony, Cannings, and Patel cases).

Meadow's evidence was flawed because of a misunderstanding of statistics—he treated each death as an independent event, an assumption which if valid, justified multiplying the accepted probability for one event (approximately 1 in 8500) to give a figure which made it appear extremely unlikely that deaths were natural (1 in 73 million). Assumption of independence is erroneous, because there may well be genetic and/or environmental factors which predispose a family to sudden infant death (‘cot death’), thus making a second death more (not less) likely.

Managing statistics in a forensic context

  • Statistical or probabilistic evidence in court should be presented by a statistician or expert with an appropriate knowledge of the subject (more generally, illustrating the danger of an expert speaking outside the area of their expertise).

  • Statistical or probabilistic evidence should never be quoted in isolation.

Sample size determination

Whenever law enforcement agencies recover suspected drugs (or, for example, DVDs containing pornographic images) it is necessary to know how many should be sampled to prove the offence. An appropriate statistical expert should be consulted for sub-sampling advice.3

Bayesian reasoning

This involves relating the probability before an observation (a priori) to that after the observation (a posteriori). Although increasingly used in forensic science, it has been criticized on the grounds that if one assumes guilt, it is easier to interpret evidence to support that assumption—the Prosecutor's fallacy (see Forensic science [link]). Bayesian thinking permits a pragmatic approach which permits investigators to focus on what are ‘more likely’ to be important pieces of evidence and disregard those less likely to be crucial.


1 Meadow R (1997). ABC of Child Abuse, 3rd edn. London: BMJ Books.

2 DiMaio DJ and DiMaio JM (1989). Forensic Pathology. St Louis: Elsevier.

3 Aitken C (1999). Sampling – How big a sample? J Forensic Sci 44:750–60.

The prosecutor's fallacy

Forensic investigation is concerned with collecting evidence for presentation to a court if called upon by either the prosecution (or plaintiff/pursuer in a civil action) or the defence. As the techniques used to collect and analyse evidence become increasingly sophisticated, it is imperative that those who use and present the evidence—the scientific experts as well as the lawyers—are aware of the increased problems with the interpretation of evidence. This is a significant issue with evidence having a statistical basis.

The good prosecutor must avoid the fallacy of the transposed conditional.

Suppose that expert evidence indicates that the probability of finding the evidence on an innocent person is very small. This might be a sample of DNA or, indeed, any other type of evidence (such as a fragment of paint, pollen, or drug) which links the suspect with the crime. The fallacy is then to assert that the probability of the accused being innocent is, therefore, correspondingly small.


The following example illustrates the fallacy. A witness to a robbery reports a man wearing a distinctive yellow jacket with a single red sleeve running from the crime scene. According to the manufacturer, only 4 jackets were manufactured like this out of a 10 000 production run. A suspect is identified and is found to own a jacket matching the description. The prosecutors fallacy asserts there is a 4 in 10 000 chance of finding this jacket type and so the probability of innocence is only 4 in 10 000. But this transposition of probabilities is exposed as nonsense if we know that all 4 jackets with this distinctive pattern were sold from the same shop in the same town. Based on such a statistic, therefore, the probability of innocence is more like 3 in 4. Note that the prosecutor's fallacy tends to rely heavily on evidence where chance occurrence is unlikely and neglects evidence that has a higher probability of happening as being less useful, even if it may help establish that a suspect was not involved.

DNA evidence is particularly prone to statistical manipulation. Suppose a suspect has a DNA match to a crime scene with a probability of 1 in 50 000, that is p=0.00002. This is certainly useful evidence, but the prosecutors fallacy is to claim that the probability of innocence is, therefore, also 1 in 50 000. Even if the population of the city is only half a million then 10 people from that geographical location could produce this DNA match. Thus, the probability of innocence, derived from this single item of evidence, is significantly larger (1 in 10).

How to avoid the fallacy

It is important to avoid either giving the impression, or the excuse, for a prosecutor to erroneously transpose the unlikeliness of an event to the unlikeliness of innocence.

It should be remembered that many strands of independent evidence properly used will reduce the likelihood of type 1 error.

It is equally important to bear in mind that the role of the medical or scientific expert is to disprove a null hypothesis, not to prove the prosecution's favoured theory.

Finding an unlikely coincidence of suspect sample and crime scene is not the same as proving guilt or establishing a lack of innocence.

The defence fallacy

In the earlier example, the defence argues that if the chance of match is 0.00002 then in a town with a population of 500 000 there are 10 possible matches. Therefore the DNA evidence is irrelevant since nine other people are also possible suspects. That is, the likelihood of guilt is only 1 in 10. The defender's fallacy is to only scrutinize the evidence piece by piece and ignore other relevant items of evidence before making a statement about the innocence of the suspect.

All the evidence should be considered, taken together. Individual pieces of evidence should never be treated in isolation.