A successful crime investigation depends upon the collection and analysis of various kinds of evidence . Forensic scientists classify evidence in different ways and have specific ways of dealing with it. One major distinction is between physical and biological evidence. Physical evidence refers to any item that comes from a nonliving origin, while biological evidence always originates from a living being. The most important kinds of physical evidence are fingerprints, tire marks, footprints, fibers , paint, and building materials . Biological evidence includes bloodstains and DNA .
Locard's Exchange Principle dictates that evidence, both physical and biological, is to be found at the scene of a crime because the perpetrator always leaves something behind by having contact with victims and objects there. Similarly, he or she will often take something away with them, which can be found on a search of their person, their garment, a vehicle, or their premises. Such evidence is often found in minute quantities and known as trace evidence . One important source of physical trace evidence is textile fibers, which usually comes from clothing or furniture involved in the crime. It may either be left behind by the perpetrator or picked up from the victim. Typically, trace evidence is invisible to the naked eye and is collected by brushing or vacuuming a suspect surface. Once collected and back in the laboratory, microscopic techniques will often be used in its examination and analysis as, for example, in the case of paint fragments or textile fibers.
Impression marks are another important kind of physical evidence. When an item like a shoe or a tire comes into contact with a soft surface, it leaves behind a pattern showing some or all of its surface characteristics, known as an impression. The collection and analysis of impression evidence found at the scene of a crime can often be very important to an investigation.
The collection of objects, marks and impressions that make up the physical evidence of a crime is a specialized task. The general principles of preserving physical evidence and assuring a secure chain of custody apply whatever the crime. However, the time and effort put into collecting evidence will be more if a serious crime, like murder or rape, is involved compared to a so-called volume crime such as burglary or car theft. In the latter case, the investigators will concentrate on the entry and exit points taken by the perpetrator where they will hope to find, above all, fingerprints and possibly tool marks.
Fingerprints are perhaps the most significant type of physical evidence in most crimes. The technology of collecting and analyzing fingerprints has been well known for over a century and has been refined over the years. A fingerprint is important as individualizing evidence. It can tie a specific person to a crime, because no two individuals have ever been found to have the same fingerprint. If a fingerprint from the scene of a crime can be linked to one in a database or from a suspect, then an identification can be made. The courts will readily accept fingerprint evidence, so long as it is properly collected and analyzed. DNA evidence, however, is rapidly becoming the gold standard of identification evidence, and when it is made less costly, will likely take over from fingerprints as the foremost manner of identification. At present, the technology is too expensive for routine use. DNA is, of course, biological rather than physical evidence.
Other kinds of physical evidence such as tire tracks and shoeprints are class evidence, rather than individualizing, evidence. This means that on its own such evidence may not be enough to convict. A shoe print taken from a relatively new shoe merely suggests the make, style, and maybe the size of a shoe. However, no shoe wears down in the same way. People walk with their own individual gait. They also take a unique path when they walk; no two people walk the same streets over time, and encounter different types of damage to the soles as they encounter the ground. Thus, over time, shoe prints may change from being class evidence to being individualizing evidence.
Class evidence such as prints from relatively new shoes or textile fibers can be valuable in identifying a suspect if taken together. A victim may have been wearing a sweater or jacket from a chain store and fibers could be found on the clothing of a suspect. If this is taken with shoe prints found at the scene from a type of trainer owned and worn by the suspect, then both items of physical evidence are strengthened and link that suspect to the crime scene.
Physical evidence can, therefore, be a highly significant part of a crime investigation. However, to play its role, the evidence must be collected and analyzed properly. In the case of a serious crime, every possible item of physical evidence must be collected. As some evidence is trace evidence, this means an extremely thorough search, or "fingertip" search, of the scene is conducted. The way this search is accomplished depends largely on the nature of the scene, but will often focus on a point such as a body, and then work outwards or inwards in a spiral. Sometimes, investigators will work in a grid formation to ensure nothing is missed. The body itself is an important source of physical evidence and a search for fibers or fingerprints will always be made before it is moved to the mortuary.
Some items of physical evidence, such as weapons, can be easy to locate and collect. However, the investigator must take care not to contaminate these items by, for instance, leaving their own fingerprints. Investigators generally cover themselves with protective clothing in order to avoid contaminating evidence at the scene. When it comes to trace evidence, other methods must be used to collect it. Hairs and fibers may stick onto a piece of sticky tape laid down on a surface. Dusting with special chemicals may reveal fingerprints or shoe prints that are otherwise invisible. Sometimes a cast is made of impression evidence like shoe prints. All the physical evidence will be photographed before anyone touches it because it is so important to keep a record of the crime scene.
It is crucial that physical evidence, whatever its nature, is not contaminated by handling. Packaging methods vary according to the nature of the evidence. Tape lifts of hair and fibers may be adhered to a piece of film and then sealed into a clean polythene bag. Fibers lifted with tweezers will be placed inside clean slips of paper called druggists' folds or bindle paper , and then sometimes sealed in plastic bags. Collection of an impression is a specialized forensic task for, unlike a hair or bullet, an impression cannot just be packaged and taken back to the lab. Impression evidence is often fragile; a tire track may deteriorate or even be destroyed by rainfall, for example.
If physical evidence is to be admissible in court, then the chain of custody must be proved. That is, each person who handled the evidence from its collection to its appearance in court must have signed for it. Therefore, the court knows who had custody of it at each stage of this journey. Precautions will have been taken to prevent any cross contamination . If someone attended the crime scene and then examined a suspect, it is possible they could transfer evidence such as textile fibers from the scene to the suspect. Ideally, the same officer would not transfer between the scene and a suspect's residence. If they do, owing to limitations on the number of personnel investigating a crime, they must undergo decontamination between locations and be able to prove to the court they have done so.
In the case of a murder, a further search for physical evidence will be made at the mortuary. Once the body is removed, the search for evidence at the scene will continue, particularly around the site where the body was found. While items of evidence are being collected, thought must also be given to collecting control samples from the scene. Thus, if chemicals have been spilled on a carpet in an incident, then it is important to have comparison samples from an unaffected piece of carpet.
Once physical evidence has arrived at the forensic laboratory, it must be stored under secure conditions. Care must be taken that items not deteriorate under their storage conditions in case there is a long interval before any criminal trial begins. There are a number of different techniques in the laboratory that can help to analyze and identify the source of physical evidence. For instance, visible microspectrophotometry is useful in identifying the chemical nature of fragments of paint or textiles. Typically, these will be compared to reference samples or to those taken from a suspect. It may be that not all the items of physical evidence will turn out to be relevant to solving a crime, but it is better the investigators collect too much physical evidence than too little. As long as they know how to keep it safe and the best way to interpret it in the context of other evidence, physical evidence can be a powerful guide as to the circumstances and perpetrator of a crime.
see also Crime scene investigation; Evidence; Paint analysis; Trace evidence.