IV. Try to retell the story about the criminal justice process

Lesson 9. PHYSICAL EVIDENCE

Classes of evidence.

Evidence can be categorized according to its potential value as proof. Certain types of evidence can be individualized, or associated with a unique source, whereas other types cannot be pinpointed but can aid in identification because they fall into particular classes. Types of evidence that can be individualized are handwriting, typewriting, fingerprints, footprints, tire marks, impressions or casts of nonstandard items, striated markings on bullets, tool marks, objects with random fractures or tears, and substances that have undergone an alteration that makes them unique among others of the same class. Among evidence that only identifies a certain class are blood and other body fluids, narcotics, toxicological materials, fibres, soils, inks, and many kinds of materials produced in batches.

Examinations can be either " unilateral " or comparative in nature. Blood-typing and establishing the distance from which a weapon was fired are unilateral examinations, whereas studies of handwriting or spent bullets involve comparisons with other examples.

Еvidence and its examination.

An investigation at the scene of a crime, particularly in crimes of violence, is the first phase of the laboratory function. Many jurisdictions have mobile crime laboratories, in which some of the regular laboratory tests can be performed by specially trained police personnel called evidence technicians.

Crime sites must be searched to locate pertinent physical evidence, which may range from latent fingerprints to bloodstains to pieces of a broken automobile headlight. Evidence must be marked for identification, preserved, and protectively packaged for transportation to the laboratory. In addition, records (written or photographic) must be made of each piece of evidence and its exact location with respect to the crime. The law requires that the " chain of evidence" remain unbroken and makes police accountable for every item of evidencefrom the time of its discovery to its ultimate presentation in court.

Certain laboratory procedures and tests are carried out in the field when time is limited or when there is a possibility that the evidencemight be disturbed. Among such procedures or tests are dusting for fingerprints, making casts of footprints and automobile tracks, making blood tests, and collecting specimens of organic and inorganic materials from the environment.

Forensic science

Forensic science has come to play an increasingly important part in the investigation of serious crimes. One of the first significant developments was identification by fingerprints. It was discovered in the 19th century that almost any contact between a finger and a fixed surface left a latent mark that could be exposed by a variety of procedures, the most common being the use of a fine powder. It was accepted in 1893, by the Troup Committee established by the Home Secretary, that no two individuals had the same fingerprints, and this proposition has never been seriously refuted. Fingerprint evidence was accepted for the first time in an English court in 1902.

The original purpose of recording and collecting fingerprints was to establish and to make

readily available the criminal record of particular offenders, but fingerprinting is now widely used as a means of identifying the perpetrators of particular offenses. Most major police forces maintain collections of fingerprints taken from known criminals at the time of their conviction, for use in identifying these individuals should they commit later crimes. Fingerprints (which may be incomplete) found at the scene of the crime are matched with fingerprints in the collection. According to the British standard, if the sets of fingerprints share at least 16 characteristics, it is considered virtually certain that they are from the same person. Searching fingerprint collections had historically been a time-consuming manual task, based on various systems of classification, but systems for electronic storage and rapid searching of fingerprint collections were developed and implemented in the 1980s.

A broad range of other scientific techniques is available to law enforcement agencies attempting to identify suspects or to establish beyond doubt the connection between a suspect and the crime in question. Examples include the analysis of bloodstains and traces of other body fluids (such as semen or spittle) that may indicate some of the characteristics of the offender. Fibres can be analyzed by microscopy or chemical analysis to show, for instance, that fibres found on the victim or at the scene of the crime are similar to those in the clothing of the suspect. Hair samples, and particularly skin cells attached to hair roots, can be compared chemically and genetically to those of the suspect. Many inorganic substances, such as glass, paper, and paint, can yield considerable information under microscopic or chemical analysis. Examination of a document in question may reveal it to be a forgery, on the evidence that the paper on which it is written was manufactured by a technique not available at the time to which it allegedly dates. The refractive index of even small particles of glass may be measured to show that a given item or fragment of glass was part of a particular batch manufactured at a particular time and place. Such information may help to identify the kind of automobile involved in a hit-and-run accident. Computer networks allow investigators to search increasingly large bodies of data on material samples, but the creation of the necessary data bases is a lengthy process.