Collecting and Preserving Evidence

Evidence aids the police in establishing the facts of a crime. Evidence is defined as any object or material
that a suspect leaves at a crime scene, or takes from the scene, or which may otherwise be connected with
the crime being investigated.

Evidence can assist an investigator by giving direction to the investigation. It aids in the solution of the case
because it can:
(a) develop or identify suspects.
(b) prove or disprove an alibi.
(c) connect or eliminate suspects.
(d) identify stolen property or contraband.
(e) help police to understand the modus operandi used to commit the crime.

NOTE: The modus operandi, or M.O., is a criminal’s method of operation. Often a particular criminal will
have a specific pattern or way of doing things that is like a “trademark.” This is the person’s M.O., and
identifying the M.O. may point to a particular person as a suspect.

In certain cases, evidence may assist in establishing that a crime was committed, and may shed light on
how a crime was committed, or who may have been involved. Below are some examples.
(a) Fibers found on a suspect’s clothes, and also present at a particular crime scene, may help to
establish that the suspect was at the scene.
(b) Footprints may show how many people were at the scene, and may also identify one or more
persons
at the scene, if the footprints can be matched to a suspect’s shoes.
(c) Automobile paint on the clothing of a body found in the road may help to establish that a victim was
hit by a car. The same evidence may later be used to help identify the specific vehicle involved.

There are two major types of evidence:

1. Specific evidence. This is defined as evidence which can be positively identified as coming from a
   specific source or person. Examples of specific evidence are fingerprints, handwriting, bullets (which can be traced to a specific gun), tool marks (which can be matched to the tool that made them), or chipped paint at the crime scene which can be matched to a specific car. DNA analysis of blood taken from a victim can be matched to blood found on a suspect’s shirt.
2. Class evidence. This type of evidence, no matter how thoroughly examined, can only be placed into a
   class or category. It cannot be positively identified as coming from a specific person or source. For example,
   a cigarette found at the scene might be shown to be the same brand as that smoked by the suspect, but this does not necessarily establish that the suspect was the one who left it there. Cloth fibers may be shown
   to match a piece of clothing a suspect wore, but it cannot be proved conclusively from this that the fibers
   came from the suspect’s clothing at the time of the crime. Soil from under a suspect’s fingernail may match the type of soil found at the crime scene, but it cannot be proved to have come from the crime scene. Class evidence is therefore a form of circumstantial evidence rather than direct evidence — it may contribute to the inference of a certain person’s guilt, but it does not prove it conclusively.

Many types of evidence may be found which are class evidence rather than specific evidence. However,
even though class evidence is not conclusive, it is still of great value to a police investigator in building a
case of innocence or guilt. This can happen when a piece of evidence has a number of class characteristics
mixed together in an unusual combination. For example, soil from a suspect’s fingernail may have traces
of fertilizer. If both the soil and the specific brand of fertilizer, in combination, appear under the suspect’s
fingernails and at the crime scene, a much stronger case of circumstantial evidence has been built.

Preservation of the crime scene is the number one priority of police officers at a crime scene. The fewer
the people at the scene, the better. Only those who are essential to the investigation should be allowed to
enter the crime scene itself. More evidence is destroyed or damaged by police officers at the scene than
any other source. If the scene is outdoors, then steps must be taken to protect evidence from the elements
– wind, rain, snow, and even direct sunlight can destroy perishable evidence. It is extremely important to
photograph evidence before removing it, and to sketch the scene so that the exact location of all evidence,
and the relative positions of different articles of evidence are preserved. In this way, the crime scene is
preserved in the form of records and diagrams long after all evidence is collected. Some forms of evidence
are particularly important. For example, blood drawn from a victim can be examined for alcohol content
even after the victim is dead. This could determine whether a person was intoxicated at the time of death,
which might be important for an accident investigation. The laboratory can figure the percentage of alcohol
in the blood by chemically separating the two substances and weighing them.

Once evidence is located, photographed and sketched, it must be collected and recorded. Anyone who
touches the evidence, whether for collection, laboratory analysis, examination, or transport to another
location, must add his name to the record of those who handled the evidence. This activity of keeping
careful account of who has had access to evidence which has been collected is known as “preserving the
chain of custody.”