For police officers around the country, the genetic profiles that 20 million people have uploaded to consumer DNA sites represent a tantalizing resource that could be used to solve cases both new and cold. But for years, the vast majority of the data have been off limits to investigators. The two largest sites, Ancestry.com and 23andMe, have long pledged to keep their users’ genetic information private, and a smaller one, GEDmatch, severely restricted police access to its records this year.

Last week, however, a Florida detective announced at a police convention that he had obtained a warrant to penetrate GEDmatch and search its full database of nearly one million users. Legal experts said that this appeared to be the first time a judge had approved such a warrant, and that the development could have profound implications for genetic privacy.

“That’s a huge game-changer,” said Erin Murphy, a law professor at New York University. “The company made a decision to keep law enforcement out, and that’s been overridden by a court. It’s a signal that no genetic information can be safe.”

DNA policy experts said the development was likely to encourage other agencies to request similar search warrants from 23andMe, which has 10 million users, and Ancestry.com, which has 15 million. If that comes to pass, the Florida judge’s decision will affect not only the users of these sites but huge swaths of the population, including those who have never taken a DNA test. That’s because this emerging forensic technique makes it possible to identify a DNA profile even through distant family relationships.