Genetic Genealogy: How Cold Cases Are Being Solved With DNA Databases

Every genetic genealogy case starts the same way: a name investigators didn't have, sitting inside DNA they'd had for years. What changed wasn't the evidence. It was where they were allowed to look for a match.

The problem it solves

Police DNA databases — CODIS in the US — only work if a suspect's DNA is already in the system, typically from a prior arrest or conviction. If the person who left DNA at a crime scene has never been arrested for anything, CODIS returns nothing, no matter how many times you resubmit the sample. That's the wall investigators hit on cold case after cold case: usable DNA, no match, decades passing.

Investigative genetic genealogy works around that wall by not looking for the suspect directly. Instead, crime-scene DNA is uploaded to a public genealogy database — most commonly GEDmatch — where people voluntarily upload their own DNA profiles to find relatives for family history research. The search doesn't need to find the suspect. It just needs to find someone distantly related to them: a third cousin, a second cousin once removed, anyone who shares enough DNA to place them on the same family tree. From there, a genealogist works backward — birth records, marriage licenses, obituaries, sometimes just Facebook — building out branches of the tree until they converge on someone who fits the case's known details: right age, right location, right window of opportunity.

Where it actually worked

The technique's breakthrough case was the Golden State Killer. Joseph DeAngelo had evaded identification for over four decades despite a huge amount of preserved evidence, because he'd never been arrested for anything that would put his DNA in CODIS. In 2018, genealogist Barbara Rae-Venter — working with retired investigator Paul Holes, who'd pursued the case for years — uploaded crime-scene DNA to GEDmatch, found distant matches, and built out family trees until the search narrowed to six possible suspects. DeAngelo, a former police officer, was one of them. He pleaded guilty to 13 murders in 2020.

That case set the template. The same technique has since been credited with helping identify suspects in hundreds of cases, including the 2022 arrest of Bryan Kohberger in the University of Idaho student murders — DNA on a knife sheath at the scene, run through genetic genealogy, pointed investigators toward him before cellphone data and vehicle sightings corroborated the match.

The part that makes people uneasy

The privacy problem with genetic genealogy isn't really about the suspect. It's about everyone else. A person who uploads their own DNA to GEDmatch to find a long-lost relative is, in effect, also making their third cousin twice removed — someone who never consented to anything — findable by police. That tension came to a head in 2019, after a Utah case in which investigators used genetic genealogy on GEDmatch to investigate a violent assault, a use the site's users hadn't been told was possible. GEDmatch changed its policy afterward to require users to actively opt in before their profile could be searched by law enforcement, which shrank the usable database overnight but gave users an actual choice in the matter. Ancestry and 23andMe, the two largest consumer DNA services, still don't allow law enforcement searches at all — which is part of why genetic genealogy cases depend so heavily on the smaller, opt-in databases rather than the biggest ones.

None of this makes the technique unusable. It does mean every genetic genealogy case is quietly running on the goodwill of people who uploaded their DNA for an entirely different reason and never expected it to end up pointing at a relative.

Hear it from the investigator's side

The BG Files, hosted by Paul Holes — the retired investigator who helped crack the Golden State Killer case — is the most direct way to hear how this actually works from someone who did it, rather than a narrator describing it secondhand.

Read about the Golden State Killer case → · Read about the Idaho student murders case → · Best cold case podcasts →