A relative’s DNA can lead investigators to a stranger

How is genetic genealogy different from a CODIS search?

The Justice Department’s interim policy says genetic genealogy differs from traditional STR typing in both technology and databases. It examines more than half a million SNPs in place of 13 to 20 STR markers, and it searches genealogy services, not CODIS. Its data cannot be uploaded to any CODIS index.¹

The policy describes forensic genealogy as “law enforcement’s use of DNA analysis combined with traditional genealogy research to generate investigative leads for unsolved violent crimes.”¹ A vendor laboratory turns the evidence sample into a SNP-based profile, which genealogy services compare by automation with the profiles of people who voluntarily submitted their own samples or profiles.¹

The service typically returns a list of users who may be related, each with an estimated relationship.¹ Closer relatives share larger blocks of identical DNA and more distant relatives shorter ones, which lets analysts make inferences about family relationships.¹ A 2018 study in Science, using genomic data from 1.28 million people tested with consumer genomics, projected that about 60 percent of searches for people of European descent would find a third cousin or closer.^2,3

Under the policy, a genetic association means that the sample’s donor may be related to a service user.¹ Law enforcement uses it “only as an investigative lead,” and traditional genealogy research and other investigative work are needed to determine its true nature.¹

How did genetic genealogy identify a suspect in 13 California murders?

Janelle Cruz was killed in Irvine, California, on May 5, 1986.^4,6,8 She was the last of 13 murder victims named in the case against Joseph DeAngelo, known in the press as the Golden State Killer.^4,5,6

In April 2018, Sacramento authorities announced that genetic genealogy had identified DeAngelo, more than three decades after the last murder.^3,4,6 Investigators said they had used a genealogy website.^3,7 News reports said the search turned up distant relatives, related at about the level of third cousins, and that investigators narrowed that family tree to DeAngelo.^3,7

Authorities said discarded DNA confirmed the identification.^5,8 He was arrested that April in Citrus Heights, in Sacramento County.^5,8 On June 29, 2020, he pleaded guilty to 13 counts of first-degree murder.^4,5 In August 2020 he was sentenced to consecutive life terms without the possibility of parole.^6,14

What did genealogy do in the Moscow, Idaho, case, and what did it not?

In the Moscow, Idaho, case, DNA from a knife sheath produced no match in CODIS. Prosecutors said the FBI used genealogy to build a family tree and sent local law enforcement a tip. They said this pointed toward a suspect but gave no substantive evidence of guilt. A direct STR comparison with his DNA then showed a statistical match, they said.^9,11,16

Kaylee Goncalves, Madison Mogen, Xana Kernodle and Ethan Chapin, students at the University of Idaho, were found dead on November 13, 2022, in an off-campus home in Moscow, Idaho.^9,12 According to the probable-cause affidavit, the Idaho State Lab found a single source of male DNA on the button snap of a knife sheath at the scene.^10,11

According to a June 2023 prosecution filing, investigators submitted the sheath’s STR profile to CODIS, and no match was found.^9,11,16 A private laboratory developed a SNP profile from the DNA, and the FBI uploaded it to one or more publicly available genealogy services.^9,11 The FBI’s family tree held hundreds of relatives, and the FBI sent local law enforcement a tip to investigate Bryan Kohberger.^9,11

On December 28, 2022, the state lab reported that DNA from trash at his family’s Pennsylvania home came from a male not excluded as the biological father of the sheath DNA’s source, the affidavit says.^9,10 Kohberger was arrested on December 30, 2022.^9,11 Prosecutors said a direct STR comparison with his cheek swab showed the sheath profile was at least 5.37 octillion times more likely to be seen if he was its source than if a randomly selected unrelated person was.^9,11,16

Prosecutors told the court the genealogy was not used to obtain any warrant and would not be used at trial.^9,15,16 In October 2023, the judge found the State’s argument that the genealogy was irrelevant “well supported,” but ruled that the defense was entitled to see at least some of it.^9,15 Kohberger pleaded guilty on July 2, 2025, to four counts of first-degree murder and one count of burglary.^12,13

What rules does the Justice Department set for genealogy searches?

The Justice Department’s interim policy took effect on November 1, 2019. It lets agencies consider genealogy for unsolved violent crimes, which it defines to include any homicide or sex crime, and to identify the remains of suspected homicide victims, but only after CODIS searches fail. It says a suspect “shall not be arrested based solely on a genetic association.”¹

The policy covers Justice Department investigations and any agency that uses Justice Department funding for genealogy work.¹ It describes itself as internal guidance that creates no rights any party can enforce against the United States.¹ Before an agency may try genealogy, the STR profile from the evidence must have been uploaded to CODIS, and CODIS searches must have failed to produce a probative and confirmed match.¹

Agencies must identify themselves as law enforcement and may search only services that give their users and the public explicit notice that law enforcement may use them.¹ A suspect identified after a genetic association must have an STR profile compared directly with the profile uploaded to CODIS.¹

Once a suspect is arrested and charged, the agency must remove the genealogy profile from the services or formally ask them to remove it.¹ In the Kohberger case, prosecutors said the profile was removed under the policy.^9,11

The policy also protects relatives who are not suspects: an agency must seek their informed consent before collecting a reference sample, unless it concludes that asking would compromise the investigation.¹ Collecting a sample covertly requires a prosecutor’s approval, and the agency must get a search warrant before a vendor laboratory analyzes it.¹ Samples and profiles may not be used to determine a donor’s genetic predisposition for disease or any other medical condition or psychological trait.¹