World of Forensic Science

Evidence is any item or information gathered at the scene of a crime, or at related locations, which is found to be relevant to an investigation. There are many different types of evidence, from DNA and tire marks, to bloodstains and fingerprints. Different kinds of evidence may require different types of expertise in interpretation. Analyzing DNA is a completely different discipline from understanding bite marks or bullet trajectories. However, there are some basic principles that apply to all forms of evidence. Perhaps the most important rule is that maintaining evidence is paramount; strict procedures must be observed by all involved in the investigation when it comes to collecting, labeling, and analyzing it. Above all, every effort must be made to ensure that evidence is not lost, damaged, or contaminated.

Evidence has many different roles in the investigation of a crime. It can link a suspect to a crime scene if, for instance, a footprint matching the shoe of the suspect is found. Evidence can also eliminate a suspect. If the shoe size of the suspect does not match that of footprint evidence, then those footprints cannot tie them to the crime scene. Evidence could also back up or contradict a witness statement, which may help guide the police in further investigations. Evidence such as DNA or fingerprints is also valuable in providing a firm identification of a perpetrator or suspect.

Forensic scientists place evidence into various categories. Direct evidence establishes fact without the need for further analysis. Perhaps the most important form of direct evidence is the eyewitness account. If someone saw a murder, then there may be nothing to add, although the witness could give false testimony and other evidence may be needed to prove this. Circumstantial evidence is more indirect and it is up to the forensic scientist to provide an explanation for it through his or her investigations. Most of the evidence handled in the forensic lab is circumstantial evidence. Although more objective than direct evidence, there is always the danger of losing or contaminating circumstantial evidence.

Forensic evidence is divided up into two basic classes, physical and biological. Physical evidence covers items of non-living origin, such as fingerprints, tire marks, footprints, fibers, paint, and building materials. Biological evidence comes from a living source, usually the victim or perpetrator. Biological evidence includes DNA extracted from blood or other bodily fluids, semen, hair, and saliva. Botanical items, such as pollen and plants, would also be considered as biological evidence. Fingerprints are probably the most valued type of physical evidence because of their ability to identify or eliminate a suspect. However, as DNA analysis technology becomes increasingly automated and rapid, it is likely that forensic investigators will place more emphasis on the collection of biological evidence.

In terms of the investigation as a whole, reconstructive evidence is relevant to understanding what actually happened at the crime scene and the sequence of events. Cast off blood, blood spatters and bullet holes can help determine exactly how the victim was attacked. Tool marks and broken glass can reveal how a perpetrator entered and left the scene. Associative evidence is used to create or eliminate a link between a suspect and a crime scene.

There are two kinds of associative evidence, class and individual. Class evidence relates to items that are, to some extent at least, mass-produced. In itself, class evidence cannot tie a crime to any one individual. For instance, a gun found at the crime scene will be of a particular make, but it will not be unique. Similarly, relatively new shoes all make similar footprints if they are the same brand and cannot be tied to any one person. However, if the shoe is worn, then the footprint may be particular to an individual, as people wear down their shoes in a unique way. Fingerprints and DNA are the most significant forms of individual evidence. In all investigations, it is individual evidence that provides the most information and is therefore, the most valued. Class evidence is also important but usually has to be taken in context with other evidence; the more, the better.

Trace evidence may fall into various categories and includes microscopic evidence such as hair, fibers, paint, and bloodstains. Locard's exchange principle explains that every contact between a suspect and people or objects at the scene of crime, including the victim, leaves traces. Evidence is transferred from suspect to scene and vice-versa. The suspect may leave their own hair behind and take seemingly invisible splashes of the victim's blood with them, for instance. Trace evidence can be a powerful form of associative evidence that can lead to identification of the perpetrator. Most often, trace evidence is found in the form of textile fibers and paint flakes.

When investigators arrive at the scene of a crime, they are faced with a wide range of evidence—from something as obvious as a body to the various kinds of trace evidence which may be present. All of it must be located, collected and packaged with the greatest care to avoid destroying or contaminated. The investigators will make a "fingertip" search to ensure that every part of the crime scene is searched for evidence. There are various ways of making this search. If a body is present, this will be searched first for trace evidence, like fibers, and swabs will be taken before it is removed. A further search will take place at the mortuary.

The investigators then might work outwards from a focal point, which could be where a body was found, or in towards it. Depending on the size and location of the scene, investigators will go over the ground in a systematic fashion in a specific pattern such as a grid or spiral. Usually two searches are carried out.

The first items to be collected are those which are fragile and could easily be damaged such as fingerprints, shoeprints, fibers, and hair. A systematic approach must be taken to ensure that the collection of one item of evidence will not destroy another. Taking casts of footprints is one example. The casting process will destroy any fingerprints present. Therefore, the location of the footprint must be dusted first for fingerprints. Some evidence may be invisible to the naked eye and may need special techniques for visualization. Luminol can reveal bloodstains and ultraviolet light shows semen stains. The investigators will also take control samples for use back in the laboratory to distinguish relevant from irrelevant evidence. For instance, if there are chemicals on a carpet, then samples of unaffected carpet must be taken for comparison. If a blanket was used to cover a corpse, then fibers must be taken to show that these do not have anything to do with the crime. The collection of individualizing evidence such as fingerprints and biological samples for DNA usually takes priority.

Each item of evidence is packaged separately to avoid contamination and damage. Every time an item is transferred from one person to another, it is signed and accounted for. The evidence is handled through a strict chain of custody, in other words. If it were not, then it could easily be challenged in court. Just one break in the chain of custody can invalidate the claim that the item of evidence was present at the scene or on the suspect and is relevant to the crime.

Of course, the forensic service does not have the resources to investigate all crimes to the extent described above. Volume crime such as burglary is distinguished from serious crime such as rape or murder. In the former case, the search for evidence may be confined to fingerprints. In the latter case, all possible evidence will be collected. The investigators cannot usually go back a second time to collect evidence. Even if it were intact, they could not prove that it had not been placed there after the crime, so it would not be admissible in a court of law. For this reason, it is generally considered important to collect too much evidence rather than too little.

The forensic scientist is charged with answering various key questions about the evidence. First, and most obvious, is identification—what the evidence actually is. On its own, however, the identity of the evidence is insufficient to shed much light on the crime. The next stage is to carry out comparison studies, using the control samples that will be collected. For instance, if bloodstains are found in the suspect's car or on their clothes, samples of blood from the victim are needed so that comparison tests can be made. These can establish whether or not the blood associated with the suspect is that of the victim. Sometimes, however, the evidence will not be of sufficient quality to allow a clear result from the comparison test. The main thrust of the forensic investigators' work is to establish links through evidence—between a suspect and a victim, place, or object. Even if there is no link, then at least the suspect can be eliminated and the investigation narrowed down. Sometimes a link can be created between a suspect and one or more places. A footprint may be found at two or more scenes, for instance. Even if there is not a suspect at this stage, the very existence of this evidence may help police know more about the suspect they are searching for, or the crime they are attempting to solve.

World of Forensic Science | Evidence