How Police Used DNA To Find The Alleged Golden State Killer


Jonathan O'Callaghan

Senior Staff Writer

Joseph James DeAngelo is suspected of being the Golden State Killer. Sacramento Police Department

Earlier this week, police arrested Joseph James DeAngelo, 72, who’s believed to be the so-called Golden State Killer – responsible for 12 murders, 51 rapes, and more than 120 burglaries. But how did they catch him?

The shocking case has spawned a huge amount of public interest. His victims include girls and women between the ages of 12 and 41, spread across central and southern California but starting in Sacramento.


The crimes took place in the 1970s and 80s, with police unable to find a suspect for decades. DeAngelo, a father and former police officer, had been hiding in plain sight living an ordinary life in Sacramento. He was arrested on Tuesday, April 24.

"We found the needle in the haystack and it was right here in Sacramento," Sacramento District Attorney Anne Marie Schubert told reporters on Wednesday. At the moment, the Sacramento Police Department has declined to reveal much information about the investigation as it is still ongoing.

Police did say they had identified DeAngelo using DNA evidence. Two previously discarded DNA samples led to the eventual match, with the help of the genealogy website GEDmatch, where people can publicly share their genetic information.

“DNA (deoxyribonucleic acid) is found in every cell in our bodies," noted National Geographic, “and, amazingly, 99.9 percent of that code is exactly the same in every human. The 0.1 percent of the code that’s different in each of us is what make us genetically unique.”


The police first linked a DNA sample collected years ago with an unidentified relative of DeAngelo on the website. After following him, they then picked up an unidentified object he discarded, which then linked him to a number of the crimes.

We share a surprising amount of DNA. ESB Professional/Shutterstock


How they actually did this is up for debate, though, as the police have not revealed detailed information. GEDmatch told The Verge that they “were not approached by law enforcement or anyone else about this case.”

One possibility is that they set up a profile on the website using DNA collected from the crime scene, and then looked for matches with any relatives. All of the data from the site appears to be publicly available, meaning they would not have needed to contact the site directly.


“No court order was needed to access that site’s large database of genetic blueprints," Paul Holes, a retired Contra Costa County District Attorney inspector, told the East Bay Times.

It raises some questions about the privacy of not just GEDmatch, but other genealogy websites like Ancestry and 23andMe. “It at once demonstrates the power of genetic genealogy research and exposes the many ethical and privacy issues,” noted The Atlantic.

Even if you’ve never used one, parts of your genome are likely available online as a result of close or distant relatives uploading their own DNA. While DNA websites like these have helped long lost relatives find each other, it can also track down people who don’t really want to be found, some who perhaps aren’t as monstrous as DeAngelo.

"If that’s how the match was obtained, then I would think there would be court battles to come," John Roman, a forensics expert from the University of Chicago, told the Sacramento Bee.


For some though, that loss of privacy is a small price to pay for bringing someone like DeAngelo to justice.


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