During the course of a complex murder investigation, hundreds of pieces of evidence may be sent to a crime lab, including the body of the victim. Investigators on the scene err on the side of caution when it comes to sending evidence to the crime lab, to ensure that nothing important is missed. Clothing worn by the victim, carpeting, any biological substances found on site, insects, a weapon, and anything unusual will be photographed on site, bagged, and sent on to the crime lab for further inspection.
From the instant that a piece of evidence is bagged, it remains in a controlled chain of custody to ensure that it is not contaminated. When it arrives at the crime lab, an officer at the lab signs for it and either locks it up or takes it immediately to a work bench for examination. Everyone who handles the evidence wears gloves, and often protective shields over garments as well to further reduce the risk of contamination.
All evidence which enters a crime lab is given a unique identity number and extensively photographed before it is examined. The initial examination of the evidence is usually paired with a dictated or written description which goes into the records associated with that evidence. Next, the evidence is dusted or fumed for fingerprints, before they can be worn away through handling. If fingerprints are found, they are lifted and set aside for cross matching.
After fingerprinting, the evidence is examined for traces of fibers or biological material like hair and blood. This evidence, in turn, will be carefully removed, bagged, and numbered for detailed analysis. Fibers will be examined and photographed, as they can provide valuable clues; if a victim was placed in the trunk of a car at some point, for example, fibers on his or her clothing could be used to determine the make of the car. Biological evidence is tested for DNA so that it can potentially be matched with a criminal or person of interest who was at the scene of the crime.
Weapons found at the scene are also carefully examined. If the weapon was a gun, ballistics tests will be performed to determine when it was fired, and to get clean samples of bullets from the weapon which could be compared with bullets on the scene of the crime, to match the weapon with the murder. Knives and other instruments are carefully tested for traces of biological material, and their shapes are photographed and described so that they can be matched with wounds on the victim. If the weapon has a serial number or unique identifier, it will be entered into a database to see if information about the weapon's most recent registered owner can be found.
The victim is taken to an autopsy suite at the crime lab to be examined by a forensic pathologist, who may find evidence on the body such as fibers, biological material, and bullets which are sent back to the crime lab. The autopsy is a very important part of the examination, because it will determine the precise cause of death, and also provide clues about the time of death. The forensic pathologist will also be able to tell if the body was moved, and identify the victim, if his or her identity has not already been established.
The analysis of evidence at a crime lab helps law enforcement track down criminals. Once someone has been arrested and formally charged, this evidence can also help to convict the accused in a court of law, if he or she is guilty. Once evidence is examined by the crime lab and shown in court, the jury decides whether it is compelling enough to merit a conviction or not.