Critics said the search was a violation of the user agreement because the case did not involve rape or murder. “GEDmatch can no longer be trusted,” wrote Judy Russell, who blogs under the name The Legal Genealogist.

GEDmatch responded by changing its terms of service yet again, expanding the list of crimes, but allowing searches only of those users who explicitly opted in.

In Texas, another GEDmatch search occurred after the police said they were looking for a sexual predator. But when a genetic genealogist gave the police a name, they charged him with burglary, raising questions yet again about whether the user agreement had been violated.

Another legal wrinkle involves the question of who is injured when the police conduct a database search. In Mr. Talbott’s case, several lawyers said, traditional legal thinking would hold that the cousins’ privacy was violated, not his.

Leah Larkin, who runs The DNA Geek, a company that helps people track down relatives, said that when the police argue that genetic genealogy is like any other tip system, they disregard the vast troves of personal information contained within a single genetic file, such as whether a suspect was born out of wedlock, was the product of incest, or carries genetic diseases.

In many states — including Washington — the police are prohibited from using Codis, the criminal DNA database, to search for relatives.

“It’s a ‘tip system’ that even convicted criminals can’t be forced to participate in,” Dr.. Larkin said. “Yet is being used on innocent people without their knowledge or consent.”