There was no match in the FBI’s national DNA database. A search of California’s forensic data bank came up blank.

The clue that led investigators this week to the door of the suspected Golden State Killer came instead from an unexpected source: an amateur genealogy website that’s something like the Wikipedia of DNA.

The arrest of Joseph James DeAngelo Jr., a 72-year-old former police officer suspected of a string of rapes and murders across California from 1976 to 1986, hinged on GEDmatch.com, an open-source platform where people volunteer their genetic information in hopes of finding long-lost family members.

The breakthrough marks a novel investigative technique concocted by authorities desperate for leads in one of the nation’s most notorious cold cases. Investigators were not sure what, if anything, they would find, but they eventually uncovered a new trail they traced back to DeAngelo’s great-great-great-grandparents. From there, they spent months narrowing down his family tree to land upon him as the suspect.

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