The Combined DNA Index System (CODIS) is the Federal Bureau of Investigation’s (FBI) program of support and software for criminal justice DNA databases. Law enforcement uses CODIS to compare DNA samples from crime scenes with the DNA of convicted criminals and arrestees, or with DNA detected at other crime scenes, in order to identify criminal suspects. The technology's analysis is limited to 13-20 designated DNA indices called microsatellites. If a match is identified in CODIS, a confirmatory sample can then be taken from the suspect. The results of a comparison between that sample and the crime scene sample can be used as evidence in court.
The existence and application of DNA databases such as CODIS have spurred debate about who should be included in DNA databases, what materials and information should be stored and the circumstances for when information should be expunged. There is also controversy surrounding the use of familial searching with partial CODIS matches, which can reveal the identity of someone with a close biological relationship to the presumed criminal suspect. Familial searching is not performed at the federal level, and the FBI does not regulate use at the state level. While Maryland and the District of Columbia have passed laws specifically prohibiting familial searching, several states have policies and procedures that allow familial searching within their databases in some circumstances.
People who view law enforcement-specific DNA databases in a positive light believe that these databases help solve serious crimes that might otherwise remain unsolved. Those who view these databases in a negative light believe some law enforcement practices within these databases violate Fourth Amendment protections against unreasonable search and seizure without probable cause and worry that the DNA samples collected might be used for unrelated purposes or to discriminate against individuals.
In 2013 in Maryland v. King,  the U.S. Supreme Court ruled in a 5-4 decision that law enforcement may collect DNA samples from suspects who are under arrest for a crime but are not yet convicted. The majority determined that the collection of DNA in such circumstances, typically via a cheek swab, is similar to fingerprinting or photographing. The dissenting justices were troubled by the implications of the decision for privacy. Justice Antonin Scalia signaled deep disagreement by reading his dissent from the bench, warning "your DNA can be taken and entered into a national database if you are ever arrested, rightly or wrongly, and for whatever reason."