How DNA evidence works in sexual assault cases
When a sexual assault crime is committed, the offender's DNA may be left on the victim's body or clothing or at the crime scene. An offender's DNA can be obtained from many different sources, including his/her saliva, sweat, blood, semen, hair, and skin cells. DNA evidence from all of these sources can be critical to identification of the offender(s), proving various elements of a crime, and successful prosecutions.
When a sexual assault victim has a forensic medical examination, the evidence collected from the victim's body and/or clothing, which may include the offender's DNA, is packaged in a sexual assault evidence kit (sometimes referred to as a "rape kit"). Additional evidence, such as body fluids left at the location of the crime (e.g., on bedding, furniture, or the rim of a drinking glass) may also be collected at the crime scene. The rape kit and the crime scene evidence samples are usually sent to a crime lab for analysis. If biological evidence is found, the crime lab attempts to obtain a DNA profile. If a profile is found, it can then be compared with a suspect's DNA sample. If there is no identified suspect, DNA offender databases that contain the archived DNA profiles of known offenders throughout the United States, as well as DNA profiles from other crime scenes in other unsolved cases, can be searched.
The database searches may produce a cold hit (the DNA from the current case matches that of a known offender whose DNA is in the database) or a case-to-case match (the DNA from the current case matches DNA in the database from one or more other unsolved crimes.
Although a case-to-case match does not result in the immediate identification of a specific suspect, it may assist in the investigations of the linked cases because information from the multiple investigations can be combined. In addition, as more offender profiles are added to the database, it may eventually be possible to identify the offender in the linked cases through a cold hit.
If no immediate cold hit or case-to-case match is found, the DNA profiles from the victim's rape kit and from the crime scene evidence remain stored in the databases and continue to be searched as the databases grow. At a future time, as more offender DNA profiles are added to the databases, the DNA profile from the current case may eventually produce a cold hit.