There are two types of trace evidence which crime scene technicians collect at the beginning of an investigation. The first is inorganic trace evidence, which includes hairs, fibers, tool marks, ballistics, glass, paint and other evidence that might be found on or around a crime scene. The second is organic trace evidence, which refers to trace evidence that comes from a human or animal.
Organic Trace Evidence: Fluids
Fluids are some of the most telling organic trace evidence because so much can be determined from their composition. Fluids left at a crime scene might include saliva, semen, sweat, and vomit, which can all be run through a large gamut of tests at the medical examiner’s laboratory.
The first test that is run on any fluids collected from a crime scene is called polymarking, which is a basic DNA test. If the person whose fluids are found at the crime scene has ever had his or her DNA recorded in law enforcement systems, or if the police already have a suspect, a DNA match will tell them whether or not their suspect is the perpetrator (or victim).
Fluids might also tell the medical examiner if the person’s whose fluids they have found has any type of disease or defect, and might also prove whether or not the person is male or female. At the crime scene, a spatter, trail or pool of fluids might give law enforcement an idea about how the crime occurred.
Organic Trace Evidence: Blood
Blood, like fluids, can tell quite a bit about the crime that occurred and about the blood left at the crime scene. Often, law enforcement will find not only the victim’s blood, but also the perpetrator’s. Blood stain analysis can help crime scene technicians to reconstruct the crime, often through photographing or “stringing”, which involves connecting string from each spatter of blood to show the sequence of events during the crime.
Blood also will give DNA evidence as well as evidence of a disease or whether the victim or perpetrator was male or female.
Organic Trace Evidence: Bite Marks
A bite mark found on the body of a victim can tell the crime scene technicians about the perpetrator. A cast is often made of bite marks to be compared with dental records in the system, and they can also tell whether the perpetrator had straight teeth, crooked teeth, crowns, expensive dental work, and a host of other evidentiary clues. Further, dried saliva and even blood is often found in the area surrounding a bite mark, which makes it much more valuable.
Organic Trace Evidence: Wounds
A wound or wound pattern will be one of the first clues about the crime, and might even betray the perpetrator’s modus operandi if he or she is a serial or pattern killer.
Most wounds, when cleaned and examined in the morgue, will tell the medical examiner a vivid story. For example, a knife wound made with a serrated blade will have a different wound chemistry than one made by a straight blade. Hesitation marks (shallow cuts to the skin before a deep cut) might suggest that the perpetrator was nervous or inexperienced.
Further, the body responds differently to trauma depending upon whether or not the victim is alive or dead when the wound is inflicted. For example, a bruise that had time to discolor or swell would indicate that the injury happened premortem, while a cut with no swelling or blood flow would indicate that the cut was made postmortem
There are other types of organic trace evidence, but these are the most common forms of organic trace evidence collected from crime scenes.