The science and profession dealing with the recognition, collection, identification, individualization, and interpretation of physical evidence, and the application of the natural sciences to law-science matters. The term for forensic science (criminalistics) originated from the book Handbuch für Untersuchungsrichter als System der Kriminalistik (3d ed., 1898) by Hans Gross, an investigating magistrate and professor of criminology at the University of Prague. He described the need for a scientifically trained investigator who could undertake certain technical aspects of an investigation and could also serve as liaison between scientific specialists who might assist in the investigation of criminal activity. This concept was popular in Europe, where forensic science institutes were developed to apply the tools and techniques of the natural sciences to the investigation of crime and, generally, in official governmental inquiries. See also: Forensic science evidence
In the United States and the United Kingdom, with legal systems fundamentally different than those of Europe, the forensic science profession has developed in a different fashion. Forensic science laboratories were established in a few police departments (in Chicago, Los Angeles, and New York) in the early decades of the twentieth century and in the Federal Bureau of Investigation (FBI). In the 1970s, an infusion of money from the federal government resulted in the formation of numerous laboratories associated with state and local law enforcement agencies.
Most forensic scientists work in laboratories associated with governmental agencies charged with enforcing local, state, or federal laws or regulations. However, some forensic scientists engage in private practice or are employed by academic institutions.
The reliability of this forensic science has been questioned, and recommendations have been made to strengthen the practice and reliability of forensic science by setting new standards, providing increased funding for research and education, developing and transferring best practices. See also: Cognitive bias in forensic science; Forensic science education; National Academy of Sciences report on forensic science
It is not possible for a single person to become proficient in the examination and analysis of all types of physical evidence. Increasingly, forensic scientists and other workers in forensic science laboratories are specializing in the examination of one or a few types of physical evidence (Fig. 1).
Forensic biology is the analysis of the biological or genetic properties of evidence. Forensic biologists are involved in the identification of biological evidence and attempts to determine its source. Traditional methods for analyzing blood-group antigens and genetically variant proteins present in blood, semen, and other biological materials have given way to the analysis of DNA that can be recovered from such materials (Fig. 2). The ability to identify the individual from whom a blood or semen sample has come has revolutionized forensic science. See also: Deoxyribonucleic acid (DNA); Forensic biology; Forensic medicine
Reference samples are routinely obtained from individuals who are incarcerated, arrested, or otherwise have come in contact with law enforcement. The collection of a reference DNA sample from inside of the cheek (generally using a swab called a buccal swab), is regulated by local, state, and national laws. Improvements in analytical techniques allow a genetic profile, often called a DNA type, to be developed from the biological material collected from items that may simply have been touched by an individual, such as a firearm used in a shooting, a soft drink can left at the scene of a burglary, or a finger smudge on the rear-view mirror of a stolen car. When a genetic profile is obtained from some object at a crime scene, the profile can be compared with profiles on file and a person may be identified.
Trace evidence analysts
These specialists analyze material that is transferred between two objects that come into contact (Fig. 3). The Locard exchange principle, credited to the French criminalist Edmond Locard, states that whenever two objects come into contact, portions (or traces) of one object will be transferred to the other. Finding these traces, identifying what they are, and determining their origin through the process of individualization (determination of the parameters of a sample that will allow it to be distinguished from other or all similar items) is the job of the trace-evidence analyst. This analysis is based on the chemical or physical properties of the material. Optical and electron microscopes and highly sensitive methods of chemical analysis (such as Fourier-transform infrared spectroscopy, energy-dispersive x-ray analysis, mass spectrometry, and neutron-activation analysis) are typically used by trace-evidence specialists. See also: Mass spectrometry; Scanning electron microscope; Spectroscopy; X-ray fluorescence analysis
Firearms and toolmark examiners
These specialists examine firearms, ammunition components, and tools and marks left by them. The underlying principle is that when a tool acts on some object it will leave a mark that is unique, due to the configuration of the cutting edge. The uniqueness of each tool is a result of manufacturing processes and postmanufacturing wear and damage. The tool may be a screwdriver that was used to pry open a door or window, or a gun barrel that produced distinctive markings on a fired bullet. The firearms and toolmark specialist will compare the marks on a recovered bullet or a toolmark on a window with guns or tools recovered during the investigation. It is often possible to conclude that a recovered bullet was fired from a particular firearm or that a mark on a window was made by a particular screwdriver. See also: Forensic firearms identification
Other evidence specialists
At a crime scene, physical evidence is routinely encountered (for example, shoe or tire impressions, gunpowder residues on the body or clothing of victims or perpetrators in shootings, fragments of bombs or destructive devices recovered from scenes of sabotage, or pieces of botanical material). There are specialists in some of these areas, but often a forensic scientist is called on to develop a method for examining unique evidence, or to consult with a scientist from an industrial or academic laboratory with expertise in an unusual field (such as forensic anthropology). See also: Forensic anthropology; Forensic botany; Forensic dentistry; Forensic engineering; Forensic entomology; Forensic mycology; Forensic physics
Forensic scientists are often involved in the analysis of suspected illegal drugs and narcotics, the examination of questioned documents, or fingerprint identification. Although they are familiar with the techniques used for the examination of these types of evidence, the examinations are usually performed by specialists. See also: Fingerprint identification; Forensic chemistry; Forensic toxicology
A wide variety of techniques are used by forensic scientists for the location and collection of evidence at crime scenes as well as for the examination and analysis of that evidence in the laboratory. Crime-scene techniques may involve the use of lasers or other light sources to locate biological stains or minute fibers or paint particles, chemical tests for lead around suspected bullet holes, electrostatic devices to recover a dusty shoe sole impression from a floor, or special reagents for the development of latent fingerprints.
Many techniques used in the forensic laboratory are the same ones that are used by analytical chemists, molecular biologists, and materials scientists. Often these techniques are adapted to the special requirements of the forensic science laboratory. Infrared spectroscopy, mass spectrometry, gas chromatography, optical and electron microscopy, and a host of other standard analytical chemistry techniques find common use by forensic scientists. See also: Analytical chemistry
Routine techniques and procedures have been developed by forensic scientists that have little or no application outside the forensic laboratory. Examples are the determination of genetic markers in minute fragments of dried biological material, the determination of the refractive index of microscopic glass fragments, the microscopic comparison of individual human hairs, and the microscopic comparison of markings on the surface of bullets. See also: Forensic microscopy
Forensic scientists require a broad education in the natural sciences, including inorganic, organic, and analytical chemistry; physics; mathematics and statistics; and biology and biochemistry. In addition, curricula in forensic science leading to bachelor's or master's degrees include courses dealing with specific types of commonly encountered evidence materials (such as dried biological fluid stains, impression evidence, or trace evidence), with specific analytical procedures (such as microscopy or instrumental analysis), and with applicable legal issues.
Because of the small number of graduates with degrees in forensic science, many forensic scientists have undergraduate or graduate degrees in scientific disciplines such as chemistry, biochemistry, genetics, or biology, and have chosen a career in forensic science after completing their education. A wide variety of in-service training is available, including training through the FBI, the California Department of Justice (through the California Criminalistics Institute), and various professional associations.