In April, a citizen scientist named Barbara Rae-Venter used a little-known genealogy website called GEDMatch to help investigators find a man they’d been looking for for nearly 40 years: The Golden State Killer. In the months since, law enforcement agencies across the country have flocked to the technique, arresting a flurry of more than 20 people tied to some of the most notorious cold cases of the last five decades.

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