SACRAMENTO — To solve a decades-old serial rape and murder case that had gone cold, investigators used DNA gathered at a crime scene and created a fake profile and pseudonym on a genealogy website several months ago, according to law enforcement officials.
An investigator with the Contra Costa County district attorney’s office and an F.B.I. lawyer worked together for several months, submitting the genetic profile of a DNA sample recovered from a 1980 murder to a genealogy website, which then delivered several matches of individuals who were distant relatives of the suspect. From there, in consultation with several genealogists, they were led to the doorstep of the man whom they believed carried out a spree of rapes and murders across California in the 1970s and 1980s.
The investigator, Paul Holes, had worked the case for more than two decades — chasing thousands of suspects and endless leads — before he looked to the nexus of genealogy and technology as a possible route to the killer.
Not everyone from the various California law enforcement agencies involved in a task force hunting the killer were sold on the idea — some raised privacy and legal concerns — but Steve Kramer, a lawyer in the Los Angeles field office of the F.B.I., agreed it had merit and was legal, Mr. Holes said in an interview.
“I was blown away with what it could do,” Mr. Holes said of the genealogy site, GEDmatch, that was used to help crack the case and identify Joseph James DeAngelo, 72, as the suspect.
GEDmatch, the genealogy site, confirmed in a statement on Friday that law enforcement officials had used its database. The F.B.I. declined to comment.
On Friday Mr. DeAngelo made his first court appearance since he was arrested.
He was wheeled into a prison courtroom in Sacramento wearing an orange jumpsuit, both wrists shackled to his wheelchair and blinking slowly with his mouth partly open.
He gazed at Judge Michael W. Sweet, who read charges that he shot to death a couple, Katie and Brian Maggiore, in Sacramento in 1978, the first of many murder cases prosecutors say he will face.
Diane Howard, a public defender who has been appointed his legal counsel, placed her hand on Mr. DeAngelo’s shoulder.
“I’m privileged to be able to represent him,” she told reporters in the foyer of the Sacramento County Main Jail. “That’s what we do.”
To find the DNA match, investigators created a fake profile, posing as a user who was researching family history. But the genetic information that was uploaded in a large text file was that of the killer. Officers found distant relatives of Mr. DeAngelo’s and, despite his years of eluding the authorities, traced their DNA to his front door.
It took about four months, from when the first possible links appeared on GEDmatch to when Mr. DeAngelo was arrested. Mr. Holes contrasted that time frame with the decades of shoe-leather police work that had always come up empty. “It underscores the power of this technology,” he said.
After working the case for 24 years, Mr. Holes was nearing retirement just as investigators were closing in on Mr. DeAngelo. Mr. Holes’s last official day on the job was March 29, three days before he drove to Mr. DeAngelo’s house — who by then was in investigators’ cross hairs but not yet under surveillance — and parked and sat there, wondering if he had finally found the killer.
After a meeting in March 2017 of investigators in the case, Mr. Holes focused his efforts on genealogy and set off on a “road show” around the state, to the various jurisdictions across California where the Golden State Killer had carried out rapes and murders.
Even as Mr. Holes pursued the novel method of online genealogy, there was still one problem: Many of the known DNA samples of the suspect had degraded over the years.
Eventually, Mr. Holes learned of a never-touched DNA sample from a 37-year-old murder in Ventura County that was sitting in a freezer. The medical examiner in Ventura at the time, Claus P. Speth, made a habit of producing duplicate rape kits, one for the investigation and one stored for later, just in case.
“It turned out to be a gold mine,” Mr. Holes said of the sample. It was given to Mr. Kramer, who had it tested in multiple F.B.I. laboratories before a genetic profile was obtained and uploaded.
It is unknown if investigators used more than one genealogy site.
Mr. DeAngelo will not be charged for a series of rapes authorities believe he committed in the Sacramento area in the late 1970s because the statute of limitations has expired, Sheriff Scott Jones of Sacramento County said in an interview on Friday. Mr. DeAngelo is being charged for several murders in Ventura, Orange and Sacramento Counties. His family, including his three children, are cooperating with authorities.
“They clearly are not complicit in any of this as far as we can tell and are as completely shocked as you might imagine, finding out your own father is a wanted prolific serial killer and rapist,” Sheriff Jones said. “I don’t think they knew.”
Mr. DeAngelo is being held in a downtown Sacramento jail, where he has been placed on suicide watch.
“That doesn’t necessarily mean he’s suicidal,” said Sheriff Jones. “It means that we and the mental health professionals that work in the jail have deemed him high risk for it.”