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“How sure are you?” the FBI agent asked Barbara Rae-Venter.

“As long as we have all the descendants in the family tree, then I’m sure,” the retired intellectual property attorney and genetic genealogist told him.

About a week later, on April 25, Joseph DeAngelo was arrested; he has since been charged with 13 counts of murder and 13 counts of kidnapping. Investigators say the 72-year-old Citrus Heights man, a former police officer, is the Golden State Killer, the notorious serial murderer and rapist responsible for a terrifying crime spree up and down the state during the 1970s and ’80s.

A key, as-yet-untold role in connecting DeAngelo to the crimes was played by Rae-Venter, who had kept quiet about her work to help solve the infamous cold case until speaking exclusively with this news organization this week. Hand-picked by Paul Holes — the retired Contra Costa District Attorney inspector who has been credited with using genealogy to catch DeAngelo — the 70-year-old Northern California resident offered critical DNA knowledge to the team of investigators leading the manhunt.

“Her expertise proved to be invaluable,” Holes said in a phone interview. “And when she was doing it, she was volunteering her time, and it just really speaks to her as a person.”

DeAngelo, who remains in custody in Sacramento County jail, has yet to enter a plea. Authorities believe he is responsible for nine rapes in Contra Costa County, and last week he was charged in four of those cases — for the crime of kidnapping during a robbery because the statute of limitations did not allow sexual assault charges. The attacks took place in Concord, San Ramon and Danville.

For Rae-Venter — who volunteers as a “search angel” for DNAadoption.com, a nonprofit that teaches adoptees to use DNA to find their birth parents — the months since DeAngelo’s arrest have been a whirlwind. She previously had used her skills mostly to find missing birth parents and researching her own family’s Kiwi roots, not serial killers.

But all that began to change in March 2015 when Rae-Venter opened an email to DNAadoption.com from a San Bernardino County detective hoping to track down the parents of an abducted child. She had no idea her involvement would eventually help solve a different, decades-old serial murder case.

‘Bear Brook Murders’

The story begins in 2003, when a man named Larry Vanner pleaded no contest to murdering his live-in girlfriend Eunsoon Jun in Richmond the year before. He had dismembered her body and buried the parts under cat litter.

A Contra Costa sheriff’s detective connected Vanner, who used many aliases, to a child abuse case where he abandoned a young girl at a Scotts Valley trailer park. The girl had thought Vanner was her father, but a blood test determined they were not related.

In 2015, the San Bernardino detective worked with the now-grown girl — a married mother of three — to find out who she was. After he sent the e-mail that connected him to Rae-Venter, the first of many dominoes fell that would eventually unravel a series of mysteries.

Making exhaustive searches that compared the woman’s DNA to genetic information posted on public genealogy websites, Rae-Venter found out the woman was Dawn Beaudin, an infant who went missing in New Hampshire in 1981. Her kidnapper, investigators then concluded, must have been Vanner.

But that was only the first revelation. After placing Vanner in the area, authorities then linked his DNA to the unsolved “Bear Brook Murders,” where a woman and three young girls were stuffed into metal drums and buried in Bear Brook State Park in New Hampshire.

Fascinated by her budding career as a gene detective, Rae-Venter continued working to learn more about Vanner. Vanner died in a High Desert Prison in Susanville in 2010, so Rae-Venter obtained his DNA from the prison autopsy, using it to find his real name, Terry Peder Rasmussen. Authorities believe Rasmussen was the perpetrator in a string of killings across the country.

Rae-Venter’s participation in that intricate case got the attention of Holes.

‘Lot of trees’

In March 2017, Holes, who had been investigating the Golden State Killer cold case, emailed Rae-Venter, saying that he needed help.

“Paul said this would really be a feather in my cap if I could help solve this one,” the soft-spoken Rae-Venter said.

By October, she was on board. She would help Holes and an FBI team from Los Angeles use the public genealogy website GEDmatch to hit pay dirt again. She didn’t know much about the Golden State Killer case but began poring over newspaper articles to develop a profile of the man to narrow down their search.

“I started doing research and was absolutely horrified,” she said.

Right off the bat, the meticulous and organized Rae-Venter saw a team of investigators winging it.

“She came in and was like, ‘No, no, no, you need to do it this way,’ ” Holes said. “She gave us structure.”

Using an untouched Golden State Killer DNA sample from a Ventura County crime lab, the FBI created a profile to load onto GEDmatch. Ultimately, the team had to dig as far back as the suspect’s great-great-great grandparents’ lineage to make progress.

“My first response was we’re gonna need to be building a lot of trees,” Rae-Venter said.

Italian heritage

Rae-Venter worked on her laptop and iPad simultaneously from her Northern California home — for safety concerns she keeps her whereabouts vague — creating a couple dozen family trees together with the rest of the team. Using knowledge of the killer’s physical attributes gleaned from his DNA left at crime scenes and other information, such as where he might have lived in order to carry out the crimes, the team began to narrow down possible suspects. DeAngelo was on the list.

“At one point she said, ‘This guy looks like he has a fair amount of Italian in him and we’re like, ‘What?!’ ” Holes said. “Nowhere had any witness accounts mentioned Italian, but when the name DeAngelo came up, we thought maybe Barbara was on to something.”

With nine names remaining, the team got a big break when a woman, who they knew likely would be a close relative, offered to be tested — she came back as a second cousin match to the killer, on his mother’s side of the family.

By early April, the list had dropped to six men, one of which was DeAngelo. The Golden State Killer DNA profile strongly indicated the man had blue eyes, so detectives ran the remaining half dozen names through the DMV database and one had blue eyes — Joseph DeAngelo.

They had their suspect.

“It’s a pretty overwhelming feeling,” Rae-Venter said. “You’ve been running on adrenaline and suddenly there’s a name popping out and it’s like, ‘Whoa!’ ”

Numb

At that point in the investigation, law enforcement took over, getting DeAngelo’s DNA to confirm the suspected match, and Rae-Venter went back to her own life. But Holes told her of DeAngelo’s arrest before the news broke.

“I guess I was still a little numb. I had helped ID him, and it was confirmed it was the right guy,” she said.

To this day, Rae-Venter has never met Holes or any of the law enforcement team in person. It was a good, new-fashioned digital manhunt.

But word moved fast among law enforcement, and amateur genetic sleuth Barbara Rae-Venter found herself very much in demand.

“I have quite a number of cases … in progress,” Rae-Venter said. “I’m hoping to get results as early as next week.”

One is the “Boy in the Box,” a 1957 cold case out of Philadelphia where an unknown boy aged 3 to 7 years old was found murdered in a cardboard box.

Most of her work is still as an uncompensated volunteer. At a recent Burbank presentation about her work, Rae-Venter realized the payoff.

A woman, who knew two Golden State Killer victims, approached her after she spoke, in tears and barely able to speak.

“Thank you! Thank you! Thank you!” she said.