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Opinion
Faye Flam

Catching a Killer in the Family Tree

Through DNA samples in online genealogy databases, police have a powerful tool to find the guilty — and exonerate the innocent.

Even if the culprit’s DNA is not on record, maybe a cousin donated at some point.

Even if the culprit’s DNA is not on record, maybe a cousin donated at some point.

Source: Science Photo Library, via Getty Images

The use of DNA from a genealogy database to identify a suspect in the Golden State Killer case marks a turning point in forensics. Now, even if DNA from a crime scene doesn’t match any known suspect or anyone whose DNA was taken on a previous arrest, detectives have another recourse: They can search for a suspect’s relatives among the millions of people submitting DNA to trace their own genealogy. The number of such samples keeps growing each year, and the more data, the greater the odds of success.

In pursuing the 20-year-old Golden State case, detectives used DNA left by the infamous California serial killer to create a profile under a fake name on a genealogy site called GEDmatch. That led them to some users who seemed to be distant relatives of the killer, and this data allowed a genetic genealogist to create a family tree. Ultimately, it pointed to a former police officer, 72-year-old Joseph James DeAngelo. He was arrested and is accused of over 50 rapes and 12 murders.