As investigators struggled for weeks to find who might have committed the brutal stabbings of four University of Idaho students in the fall of 2022, they were focused on a key piece of evidence: DNA on a knife sheath that was found at the scene of the crime.

At first they tried checking the DNA with law enforcement databases, but that did not provide a hit. They turned next to the more expansive DNA profiles available in some consumer databases in which users had consented to law enforcement possibly using their information, but that also did not lead to answers.

F.B.I. investigators then went a step further, according to newly released testimony, comparing the DNA profile from the knife sheath with two databases that law enforcement officials are not supposed to tap: GEDmatch and MyHeritage.

It was a decision that appears to have violated key parameters of a Justice Department policy that calls for investigators to operate only in DNA databases “that provide explicit notice to their service users and the public that law enforcement may use their service sites.”

It also seems to have produced results: Days after the F.B.I.’s investigative genetic genealogy team began working with the DNA profiles, it landed on someone who had not been on anyone’s radar: Bryan Kohberger, a Ph.D. student in criminology who has now been charged with the murders.

The case has shown both the promise and the unregulated power of genetic technology in an era in which millions of people willingly contribute their DNA profiles to recreational databases, often to hunt for relatives. In the past, law enforcement officials would need to find a direct match between DNA at the crime scene and that of a specific suspect. Now, investigators can use consumer DNA data to build family trees that can zero in on a person of interest — within certain policy limits.