Both companies require a court order for access and say that they have not yet permitted law enforcement to conduct a genetic search. But interest is high. Eric Heath, Ancestry’s chief privacy officer, said in an interview last month that he received 24 emails in 2019 requesting access to the site from law enforcement. These emails included a request to upload DNA to try to identify a suspect in a cold case and a request to search for relatives of an unidentified body. Mr. Heath said he responded by sharing the site’s policy and the requests ended there.

But they may not in the future. In July, a judge in Florida granted a detective a search warrant to obtain access to nearly 1 million GEDmatch users (more on GEDmatch below) who had not elected to help law enforcement. Many privacy advocates and genealogists were horrified and warned that the development would encourage warrants for searching bigger sites like Ancestry.

Anne Marie Schubert, the Sacramento County district attorney, has been involved in advising law enforcement agencies on how to solve crimes with genealogy sites since her agency helped crack the Golden State Killer case. She said she supported the judge’s ruling. “I commend Florida for taking that first step,” she said, calling it “a natural progression in an evolving world.” She said she believes that investigators will eventually gain access to Ancestry.

Should such a search warrant be served to Ancestry, Mr. Heath said, the company is prepared to fight. “We’re willing to push back and narrow the scope and squash it however we need to,” he said. 23andMe made a similar commitment on its blog.

But what if I want to help catch killers?

Ms. Schubert is among a group of people who are encouraging the big companies to allow users to opt in to help law enforcement. Mr. Heath said that won’t happen. Other databases serve that purpose, he said, adding, “I don’t want to manage that.”

GEDmatch

How big is the database?

1.3 million DNA profiles

What is the point of this site?

It is unlikely that anyone will be getting you the gift of a GEDmatch subscription. Uploading to the site is free and the company does not offer DNA tests. But when uploads to other sites rise, uploads to GEDmatch typically follow. That’s because the site functions as a means to get more from your DNA; you can take a DNA file analyzed by another company, like 23andMe, and upload it to GEDmatch to find more relatives and ancestry data.

If I join, could the police use my DNA?

Short answer: Maybe.

Longer answer: People who upload to GEDmatch can choose among four settings. These include help law enforcement, opt out of law enforcement searches, and research mode, which is supposed to hide your profile from everyone.