Law enforcement officials often turn to DNA identification methods to detect—and rule out—possible offenders. Every state operates its own database of convicted offenders’ DNA profiles; some states store profiles of arrested people, too. The Federal Bureau of Investigation maintains a national database of profiles submitted by laboratories across the country.

A few years ago, officials came up with a new way to use DNA profiles in forensic identification. Ordinary searches require an exact match between DNA found at a crime scene and a forensic DNA profile. A partial match means that the profiled individual should not be considered a suspect. But partial matches create another possibility: the crime scene DNA may come from a relative of the individual whose profile is in the database.

The United Kingdom has used partial matches to identify unknown offenders in several high-profile cases, and the technique is gaining traction in the United States. The 2010 identification of a suspected serial killer could accelerate this trend.