What Every Law Enforcement Officer Should Know About DNA Evidence
National Institute of Justice
Introduction
Just as today’s law enforcement officer has learned to look routinely for fingerprints to identify the perpetrator of a crime, that same officer needs to think routinely about evidence that may contain DNA. Recent advancements in DNA technology are enabling law enforcement officers to solve cases previously thought to be unsolvable. Today, investigators with a fundamental knowledge of how to identify, preserve, and collect DNA evidence properly can solve cases in ways previously seen only on television. Evidence invisible to the naked eye can be the key to solving a residential burglary, sexual assault, or child’s murder. It also can be the evidence that links different crime scenes to each other in a small town, within a single State, or even across the Nation. The saliva on the stamp of a stalker's threatening letter or the skin cells shed on a ligature of a strangled victim can be compared with a suspect's blood or saliva sample. Similarly, DNA collected from the perspiration on a baseball cap discarded by a rapist at one crime scene can be compared with DNA in the saliva swabbed from the bite mark on a different rape victim.
Similar to fingerprints
DNA is similar to fingerprint analysis in how matches are deter- mined. When using either DNA or a fingerprint to identify a suspect, the evidence collected from the crime scene is com- pared with the “known” print. If enough of the identifying fea- tures are the same, the DNA or fingerprint is determined to be a match. If, however, even one feature of the DNA or fingerprint is different, it is determined not to have come from that suspect. This brochure will explain DNA and the related identification, preservation, and collection issues that every law enforcement officer should know.