The FBI’s Combined DNA Index System (CODIS) is a searchable computer database of DNA profiles from arrestees/convicted offenders (guidelines determined by each state) and DNA profiles found at crime scenes. There are other types of DNA profiles within CODIS pertaining to missing persons investigations as well. It is a tool with which unsolved crimes can be aided by a ‘cold hit’.
Prior to CODIS, DNA analysis of cases was only meaningful if DNA from possible suspects was also available for comparison to the evidence. In the era of CODIS suspectless cases can be analyzed and any probative DNA profiles are entered into the database to search against DNA profiles of individuals and other cases. If a hit does not occur, the DNA profiles from the case will remain in the database for comparison to future entries.
Unsolved cases may hit on each other in CODIS as well. If Police Department A enters a semen DNA profile from a suspectless case and Police Department B does the same for another suspectless case, the Departments will be alerted if the DNA in their cases matches. Although the perpetrator has not been identified, Police Department A and Police Department B can now assist each other in their investigations with the knowledge that they are likely chasing the same individual.
In recent years, the concept of familial searching has been applied to CODIS. If a direct hit between an arrestee/convicted offender and a DNA profile from evidence does not result, but there is significant similarity between the two, it may be a clue that the real perpetrator is a family member of the arrestee/convicted offender. The more closely individuals are related, the more similar their DNA profiles are. The first use of familial searching to obtain an arrest in a homicide case in the U.S. was the ‘Grim Sleeper’ case in Los Angeles.