How DNA led to the elusive ‘Golden State Killer’

The arrest this week of 72-year-old Joseph James DeAngelo — believed to be the “Golden State Killer” responsible for 12 murders and more than 50 rapes in the 1970s and ’80s — was hailed as a victory for cutting-edge science and old fashioned detective work.

“The answer was, and always was going to be, in the DNA,” said Sacramento County District Attorney Anne Marie Schubert.

Here’s how it unfolded.

– Crime scene DNA –

Schubert opened a cold case investigation into the Golden State Killer two years ago, according to the New York Times.

Investigators started with what Schubert called “abandoned” DNA samples left at crime scenes, though officials haven’t said what the samples entailed.

“You leave your DNA in a place that is a public domain,” she said.

This discarded DNA allowed experts to compile a genetic profile of the suspected attacker.

– Relative on ancestry site –

Detectives uploaded the suspect’s genetic profile into online genealogy databases, to see if they could find a match.

The FBI wanted poster for the 'Golden State Killer'

Although DeAngelo himself had not sent his own DNA to any of these sites, at least one distant relative of his had, possibly more.

A Lake Worth, Florida-based company called GEDMatch acknowledged on Friday that their database “was used to help identify the Golden State Killer… although we were not approached by law enforcement or anyone else about this case or about the DNA.”

The company warned customers in a statement that even though the site was intended for genealogical research, possible uses of their DNA include “identification of relatives that have committed crimes or were victims of crimes,” and they should delete their profiles if they had any concerns.

– Online family trees –

The crime lab began exploring online family trees that appeared to match the suspect’s DNA profile.

They hunted for clues about various individuals in those families, to see if they were possible suspects.

On April 19, detectives decided that DeAngelo might be the one because a number of factors aligned: the DNA, his age, and the fact that he lived in the area where the crimes occurred.

Investigators set up surveillance in the tree-lined suburb where DeAngelo lived, and collected something he discarded in order to obtain DNA.

Officials have not said what that item was, but it could have been a soda can, a hairbrush, or anything containing DeAngelo’s saliva, hair or blood.

They compared the newly collected DNA sample to the old DNA from the crime scene, and it was a match.

The sample provided “overwhelming evidence that it was him,” Schubert said, according to the Sacramento Bee.

– ‘Astronomical evidence’ –

She asked the sheriff’s office to collect a second sample, to be sure. So they did.

“The second sample was astronomical evidence that it was him,” Schubert said.

A Sacramento County sheriff's deputy stands outside the home of accused 'Golden State Kill'Joseph James DeAngelo in Citrus Heights, California

DeAngelo was arrested outside his home Tuesday and charged with murdering two people in 1978 in Rancho Cordova, California.

He is expected to face more charges.

“This was a true convergence of emerging technology and dogged determination by detectives,” Sacramento County Sheriff Scott Jones said.