GEDmatch said in a statement that law enforcement never approached it about the case in California.

“It has always been GEDmatch’s policy to inform users that the database could be used for other uses, as set forth in the site policy,” the website said, adding that participants’ information could help in the “identification of relatives that have committed crimes or were victims of crimes.”

Paul Holes, a recently retired cold-case detective with the Contra Costa County Sheriff’s Office, described the process of narrowing down suspects using DNA.

“When you find somebody that has DNA that they might share with our offender … then you find somebody else. And if you see that they share a little more DNA, you’ve stepped a bit closer to who the offender is,” Holes told NBC News. “And so you end up marching down that path until, ultimately, you get within a reasonably small suspect pool.”

The pool in this case included DeAngelo, who was the right age and lived in the area where many of the crimes took place, officials said. Investigators kept tabs on him for six days, collecting actual DNA on items he had thrown out, before arresting him Tuesday night at his home.

And DNA potentially may have played an earlier role in the case: by stopping the crime spree. Genetic testing was just coming into use as a criminal investigative tool in 1986 when the Golden State Killer apparently ended his decadelong wave of attacks.

DeAngelo, who was a police officer for two departments in the 1970s, most likely would have known about the new method, experts said.

After Sacramento County prosecutors confirmed Thursday that they found DeAngelo through genealogical websites, Ancestry.com and 23andMe issued statements denying that they had played any role.

Still, privacy laws aren’t strong enough to stop other police departments from prying, said Steve Mercer, the chief attorney for the forensic division of the Maryland Office of the Public Defender.

“People who submit DNA for ancestors’ testing are unwittingly becoming genetic informants on their innocent family,” said Mercer, adding that they “have fewer privacy protections than convicted offenders whose DNA is contained in regulated data banks.”

Sheldon Krimsky, a Tufts University professor who studies ethics in science and medicine, said almost half of the firms that provide ancestry information will sell customers’ genetic information to some other company. Those might include pharmaceutical or drug developers that want it for research.

Earlier this year, Krimsky said in an interview with Tufts Now, a news site affiliated with Tufts University, that only 10 percent of the ancestry companies will destroy a person’s original sample.

“The vast majority hold onto your sample or sell it. So it’s not just the data, but your actual saliva, that’s being shopped around,” he added.