In law, some cases are never prosecuted because no suspect is ever located. In other cases, there are suspects but there is not enough evidence to prosecute. Cases that remain unsolved are called cold cases.
Cold cases are often handled by special divisions in law enforcement. They may for example begin to examine a cold case when new evidence has been presented; there may be an individual that decided to come forward with evidence or with a witness testimony. Those types of evidence may be enough to reopen a cold case.
Cold cases may also be check and reviewed periodically, depending on the jurisdiction. Certain types of cases tend to be reviewed more frequently, such as missing children. Cases may be reviewed once evidence in another case matches the scenario in the cold case, such as the manner in which a new crime was committed.