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The day after law enforcement officials announced that they had finally arrested the Golden State Killer, the focus turned to the question of “how?”

As my colleague Thomas Fuller put it, “it was technology that got him.” (You can read his article here.)

By the late afternoon on Thursday, officials at the Sacramento district attorney’s office were delving further into exactly how they traced the man they accuse of committing more than 50 rapes and 12 murders. The suspect, Joseph James DeAngelo, 72, was arrested by the police this week.

Investigators, it turned out, had used DNA from crime scenes that had been stored over the decades and plugged the genetic profile of the suspected assailant into an online genealogy database. Eventually, they traced the DNA to the suspect’s front door.