DeAngelo, a former cop himself, is thought to be the criminal known variously as the East Area Rapist, Golden State Killer and Visalia Ransacker. While most people are naturally glad the 72-year-old was eventually caught after allegedly killing 12 people and raping 50 between 1974 and 1986, the case does highlight how much can be revealed through consumer DNA tests—not only about the person having their DNA tested, but about their relatives too.

DeAngelo wasn’t caught because he sent his own DNA swab off to a genealogy site for analysis, but because someone related to him did.

The investigators took a DNA sample from one of his many suspected crime scenes, and compared it with the information stored on genealogy sites, many of which offer DNA testing as part of their portfolio of services.

We all share genes with our relatives, in diminishing amounts as the relationship gets more distant—you share around half your genes with each parent and sibling, 12.5% with your first cousin, and so on.

So, when the investigators found a match between the sample they had and a user of the genealogy site, they then searched the user’s family tree for people who might be potential suspects, based on clues in the case. Once they established DeAngelo as a firm suspect, an investigator got his DNA from something he discarded while under surveillance, tested it against the DNA from the crime scene, and found what Sacramento district attorney Anne Marie Schubert told reporters was “overwhelming evidence that it was him.”

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The Federal Trade Commission warned consumers several months ago that the increasing popularity of DNA testing kits comes with privacy implications. “The data can be very enlightening personally, but a major concern for consumers should be who else could have access to information about your heritage and your health,” the FTC said. “If you’re thinking about buying an at-home DNA test kit, you owe it to yourself–and to family members who could be affected–to investigate the options thoroughly.”

The Sacramento DA’s office did not identify the site or sites involved in the case, but 23andMe and Ancestry, two of the most popular consumer DNA-testing services, told Mashable it wasn’t them.

23andMe said it would resist law enforcement inquiries in order to protect privacy, and has never shared genetic investigation with the cops. Ancestry, which has in the past helped investigators in a murder and rape case in Idaho, said it would only share information with investigators that follow “valid legal process.”