DNA’s identity crisis

In the fall of 2006, a grisly murder case landed on Bicka Barlow’s desk, one that had gone unsolved for more than 30 years. As the resident DNA expert in the San Francisco Public Defender’s Office, Barlow handles the genetic evidence for most files that come through the door. The accused, John Puckett, was an ex-felon who, by all accounts, had been living a quiet life for the past two decades. He was an old man by now, and his health was failing; if convicted, he would probably die in prison.

The California Department of Justice had identified Puckett through a “cold hit,” in which his DNA was matched to the long-ago crime scene through a search of the state’s criminal offender database. The San Francisco Police Department was extremely excited about the case: It was the second-oldest cold case in city history, and solving it was made possible, a triumphal press release declared, by advances in DNA technology and a state grant aimed at increasing cold-hit prosecutions. After examining the particulars of the case, Barlow realized this just...