The word pierced a momentary hush that had settled over the packed courtroom, where a line of people stretched out into the hall. The word that the families of Jay Cook and Tanya Van Cuylenborg had been waiting for more than three decades to hear: “guilty.” After a day and a half of deliberations, a Snohomish County jury found William Earl Talbott II guilty on two counts of aggravated murder in the first degree for the deaths of the young Canadian couple. They disappeared during an overnight trip to Seattle on November 18, 1987—their bodies recovered in rural western Washington a few days later, each bearing the marks of the violent ends they met.

The decision, delivered late Friday morning, is a momentous victory for a new kind of genetic detective work that builds on the spit kit results of genealogy-curious relatives.

It cracked open the Snohomish County case when investigators worked with a private company to trace semen left at one of the crime scenes to Talbott through two cousins who had uploaded their own genetic information to a public database called GEDMatch. Such sites help people build family trees and trace their ancestral roots through shared DNA, a practice known as genetic genealogy. Recently, police departments and the FBI have begun using the method to dig up suspects. Genetic genealogy has already been used to advance dozens of cold cases since it very publicly resulted in the arrest of the suspected Golden State Killer last April. But until Talbott’s case went to trial at the beginning of this month, the technique had never been tested in court.

The Talbott verdict is the first 12-person vote of confidence in genetic genealogy’s ability to not just put a name to a drop of blood or skin cells lifted from a fingerprint or a semen-soaked swab, but to help prosecutors prove that the person behind that name also committed the crime they’ve been accused of. It’s an outcome that could favorably impact other cases currently making their way to court in other states, even if just to bolster public opinion.

But even more significantly, the image of Talbott gasping as the jury read out their verdict signals to law enforcement agencies all over the country that genetic genealogy is good for more than just generating leads and making arrests, it’s good for getting convictions too. And that has huge implications for both the future of crime-fighting and genetic privacy.

"We didn't know whether juries might be skeptical of evidence that police came about through this means, or whether they'd think maybe that it's a problematic investigative tool" says Andrea Roth, director of UC Berkeley’s Center for Law and Technology. "The main thing this teaches us that we didn't know yesterday is that you can convince a lay jury to convict someone found as a suspect through genetic genealogy."

Roth and other legal experts have raised concerns that this new phenomenon of police rifling through non-criminal databases violates principles of informed consent and threatens citizens’ constitutional protections against warrantless searches. DNA isn’t like a social security number. It’s an inherited substance, a code you share (to differing degrees) with all your biological relatives. So when people agree to let law enforcement access bits of their genome, they’re getting bits of yours too. Even if you never take a DNA test yourself.

Researchers estimate it will be just a few years before every white person in America can be identified this way. And unlike the kinds of DNA technologies police have been using for decades to match crime scene samples to suspects, the genetic profiles generated for genealogy purposes hold a lot more information—including sensitive health information.