Contaminated evidence

Contamination is the introduction of something to a scene that was not previously there. This means trace materials are added to a crime scene after the crime is committed. This can happen before, during and after authorities take samples of the evidence from a scene. Many people can contaminate the evidence at a crime scene, including witnesses, suspects, victims, emergency response personnel, fire fighters, police officers and crime scene investigators. Juries expect to see forensic evidence before they make a decision in a case which relies on that evidence. Because of this, attorneys on both sides try to discredit forensic evidence that does not support their clients' interests. This requires crime scene investigators be especially careful to guard against contamination in the forensic samples taken from a crime scene. A miscarriage of justice can occur when these procedures are not carried out carefully and accurately.

Evaluating a scene before anyone enters can be key to keeping contamination to a minimum. The examination of the scene will usually begin with a walk through of the area along the "trail" of the crime. The trail is that area which all apparent actions associated with the crime took place. The trail is usually marked by the presence of physical evidence. This may include the point of entry, the location of the crime, areas where a suspect may have cleaned up, and the point of exit. In some cases, a walk through may become secondary if potential evidence is in danger of being destroyed. In that case, this evidence should be preserved, or documented and collected as quickly as possible.

The purpose of the walk through is to note the location of potential evidence and to mentally outline how the scene will be examined. The walk through begins as close to the point of entry as possible. The first place the investigators should examine is the ground on which they are about to tread. If any evidence is observed, then a marker should be placed at the location as a warning to others not to step on the item of interest. As the walk through progresses, the investigators should make sure their hands are occupied by either carrying notebooks, flashlights, pens, etc. or by keeping them in their pockets. This is to prevent depositing of unwanted fingerprints at the scene. As a final note on the walk through, the investigators should examine whatever is over their heads (ceiling, tree branches, etc.). These areas may yield such valuable evidence as blood spatters and bullet holes. Once the walk through is completed, the scene should be documented with videotape, photographs, and/or sketches.

The most important aspect of evidence collection and preservation is protecting the crime scene. This is to keep the pertinent evidence uncontaminated until it can be recorded and collected. Eating, drinking, or smoking should never be allowed at a crime scene. A command post should be set up for such purposes. The post is to be set up somewhere outside the restricted areas. It could be a vehicle, picnic table, hotel room, tent, etc. It can be used as a gathering place for non-involved personnel, a place for investigators to take breaks, eat, drink, or smoke, a communication center, a place for press conferences, a central intelligence area, etc. Protection of the crime scene also includes protection of the crime scene investigators. One person, whether a civilian or a police crime scene investigator, should never be left alone while processing the scene. This is especially true if the suspect has not been apprehended. There are many stories of suspects still hiding at or near their area of misdeed. That is why there should always be at least two people working the scene. At least one of these people should have a radio and a firearm.

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Wikipedia