At the same time, DNA from the scene was being processed. Forensic teams had examined the knife sheath and found DNA that did not belong to any of the inhabitants of the house. They ran the sample through the F.B.I.’s database, which contains millions of DNA profiles of past criminal offenders, but according to three people briefed on the case, they did not get a match.

At that point, investigators decided to try genetic genealogy, a method that until now has been used primarily to solve cold cases, not active murder investigations. Among the growing number of genealogy websites that help people trace their ancestors and relatives via their own DNA, some allow users to select an option that permits law enforcement to compare crime scene DNA samples against the websites’ data.

A distant cousin who has opted into the system can help investigators building a family tree from crime scene DNA to triangulate and identify a potential perpetrator of a crime. In one of the best-known uses of genealogy in solving a cold case, detectives in 2018 were able to arrest a suspect in a string of rapes and murders years before in California that had been attributed to the so-called Golden State Killer.

Once a suspect is identified, a direct genetic comparison can be used as confirmation before making an arrest.

With few formal regulations, the use of mass genealogy databases has raised concerns about privacy and whether limits should be placed on how the method is used. But authorities across the country say they have found potential in the system to produce leads that are unattainable through traditional investigative efforts.