7. Trace evidence

Fibres, hairs, paint, glass, and explosives traces are examined by forensic scientists in a wide range of crimes from burglary to terrorism. The distinctive characteristics of trace evidence are its microscopic size or minute amount, its ability to transfer readily from one item to another, and that it is subsequently lost from the item following that transfer. Many of the difficulties that arise in the examination, analysis, and interpretation of trace evidence are consequences of these characteristics. ‘Trace evidence’ considers the principles that underlie trace evidence examination, some of the scientific techniques used, how the significance of the evidence is assessed, and the importance of trace evidence in police investigations.

6. Prints and marks: more ways to identify people and things

‘Prints and marks: more ways to identify people and things’ uses fingerprints and shoe marks to illustrate the general characteristics of marks evidence, the principles involved in their examination, and how the evidence is evaluated. Marks can be visible (patent) or invisible (latent), and require specialist optical, physical, or chemical techniques to visualize them. They can be made in a variety of substances such as mud, blood, dust, and sweat (referred to as ‘negative’ marks) or by transferring a material to another surface (‘positive’ marks). Fingerprints are a reliable means of identification, but there are important issues about procedures, standards, and the training of experts that still need to be addressed.

3. Crime scene management and forensic investigation

‘Crime scene management and forensic investigation’ shows how the actions of investigating officers at a crime scene can affect the availability and efficacy of forensic tests later in the investigation. The purpose of crime scene management is to control, preserve, record, and recover evidence and intelligence from the scene of an incident in accordance with legal requirements and to appropriate professional and ethical standards. Any items removed from a scene by investigators must be packaged and labelled correctly to prevent contamination and minimize damage. Once forensic analysis begins, it is essential to consider the investigative implications of actions and decisions. It is sometimes more effective, quicker, or convenient to bring the specialist to the crime scene.

5. DNA profiling and databases

‘DNA profiling and databases’ looks at the biological basis of DNA profiling and how DNA is analysed and interpreted in different case types. It first explains the structure of DNA and the process of extraction and purification of the DNA. The analysis, interpretation, and evaluation of DNA evidence are then discussed along with the use of DNA databases in the investigation of crime. The impact of DNA profiling has been immense because it can eliminate or identify an individual from minute traces with great confidence. However, the ultimate meaning of any DNA evidence depends not only on the experts, but those who are called to adjudicate the evidence of a case as a whole.

2. Investigating crime

‘Investigating crime’ explains how the police investigate criminal activity, setting out some of the principles and procedures involved and how forensic science provides them with answers. Radical changes in English law and forensic science practices resulted in the introduction of formal quality assurance systems to standardize laboratory practices and the Police and Criminal Evidence Act (1984), which specifies how police deal with accused persons, go about their investigations, and are accountable for their actions. The ‘trace, interview, eliminate’ (TIE) method of systematic elimination of individuals from an investigation is described, as well as the role of digital forensics to extract and interpret data from a wide range of devices.

8. Drugs and toxicology

‘Drugs and toxicology’ provides an overview of the range and complexity of forensic drug examination. The work of a forensic toxicologist involves the analysis of body fluids and tissues to determine the presence or absence of drugs in the living or the dead. The effects of drugs such as cannabis, heroin, new psychoactive substances, GHB, and alcohol are considered. Following an initial drug screening, the most common analytical techniques for identifying and quantifying drugs are various combinations of liquid chromatography and gas chromatography with mass spectrometry. Once a drug has been identified, the next question that arises is: what is the significance of this?

4. Laboratory examination: search, recovery, analysis

‘Laboratory investigation: search, recovery, analysis’ outlines the various stages of recovering, documenting, and analysing evidence. It describes the types of examinations carried out, different laboratory structures, the range of analytic methods available, and the specific scientific and legal procedures required to meet the standards of criminal law. The importance of minimizing contamination, maintaining continuity (chain of custody), and quality assurance are also explained. Locating and identifying blood, semen, saliva, and other biological materials form a routine aspect of forensic biology. It is important when performing and recording laboratory investigation to separate fact (e.g. from test results) from the forensic scientist’s own opinion.

9. Science and justice—a case study

The defining feature of forensic science is its relationship with the law. ‘Science and justice—a case study’ considers the final stages of a criminal inquiry—adjudication by the courts. The significance of forensic evidence is assessed by human cognition. The evidence is weighed carefully by experts, but words can be misunderstood or manipulated. The murder of Jill Dando in 1999 is used as a case study, where the significance of gunshot residue became a critical part of the case. The canon of forensic technologies will continue to develop and change. The conceptual gap between technology and the law is likely to remain and perhaps increase.

1. What is forensic science?

‘What is forensic science?’ describes forensic science as the investigation, explanation, and evaluation of events of legal relevance including the identity, origin, and life history, and interaction of humans, materials, substances, and artefacts. A variety of techniques and methodologies, some of which are scientific, are used to describe, infer, and reconstruct events based on the analysis and evaluation of fragmentary physical evidence and other relevant information. From these facts, established to some pre-determined legal standard, the law infers behaviour, motivation, and criminal intent. In short, forensic science answers the central questions in a criminal investigation: who, what, where, when, why, and how?