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It was a Monday afternoon in 1988 when a co-worker found 29-year-old Diane Lynn Dahn bludgeoned and stabbed to death in her apartment in Santee. Wandering nearby was Dahn’s 2-year-old son.

The case went unsolved for 34 years.

Now, Dahn’s son, Mark Beyer, 36, says he finally has closure.

On Wednesday, sheriff’s officials announced they had a suspect. They used DNA and a process known as genetic genealogy to identify the accused killer: Warren Robertson.

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Robertson died in a house fire in Indiana in 1999. He was 39.

Mike Beyer, Diane Dohn's son, speaks via a video during a press conference Wednesday.

Mike Beyer, Diane Dohn’s son, speaks via a video during a press conference Wednesday, when sheriff’s investigators announced they solved Dahn’s murder through genetic genealogy.

(Sandy Huffaker/For The San Diego Union-Tribune)

“The answers that my family received — it’s closure, and closure is everything, even after so much time had passed,” Beyer said in a video officials showed during a news conference.

According to investigators, Robertson and Dahn lived in the same apartment complex on Graves Avenue near Prospect Avenue in 1988. Robertson was a tow truck driver; Dahn worked as an electronics technician with San Diego Transit Corp. Both were racing enthusiasts who attended stock car races.

Investigators were unable to determine if they knew each other before the slaying.

Also unknown is the motive. Although Dahn was nude when her body was found, investigators said there was no evidence of sexual assault. They said Robertson had a criminal history that included low-level property crimes but no violent crimes.

Investigators said Robertson moved to Lakeside soon after the homicide, then to Indiana the following year.

For years, investigators tried to use DNA to solve the case, to no avail. In late 2000, they extracted DNA from samples that had been scraped from Dahn’s fingernails. When they uploaded the DNA to a national criminal database, there was no match.

In the summer of 2010, investigators turned to new technology to process a hair follicle that had been found in Dahn’s hand. Once again, they uploaded the DNA to the national database — and again, they found no match.

It was in May 2020 that investigators turned to genetic genealogy. They used genealogy sites to find relatives whose DNA matched the DNA of the unidentified suspect. According to investigators, the Sheriff’s Department uses websites that give users the option to allow law enforcement to view their DNA profiles to find matches.

In Dahn’s case, there were many matches. The next step was to build family trees to zero-in on the suspect. Throughout the process, investigators contacted relatives to ask for help to build out the branches of the family trees. Investigators would construct nine family trees, with nearly 1,300 relatives tied to Robertson either through blood or marriage.

“The process was extremely laborious,” said Jeffrey Vandersip, senior crime intelligence analyst.

The genealogy led investigators to Robertson’s children. A paternity test confirmed that the DNA investigators had matched Robertson’s DNA.

“This case would have never been solved without genetic genealogy,” Vandersip said. “I have no doubt in my mind.”

Victoria Dohn-Minter, Diane Dohn's sister, speaks during a press conference Wednesday.

Victoria Dohn-Minter, Diane Dohn’s sister, speaks during a press conference Wednesday.

(Sandy Huffaker/For The San Diego Union-Tribune)

Dahn’s sister, Victoria Dahn-Minter, thanked investigators for their work on the case over the years.

“I didn’t think anything was ever going to come of this,” Dahn-Minter said through tears. “I thought I, myself, was going to go to my grave not knowing (who killed Dahn).”

She said her older sister taught her music and foreign languages, and recalled ditching school together to go skiing.

“We just did lots of things together,” Dahn-Minter said. “We always wanted to raise our children together. When this happened, it was just devastating.”

The day she got the news that the case was solved “probably was — other than the birth of my children — the best day of my life,” she said.

Beyer, Dahn’s son, said a family friend adopted him after his biological mother’s death. He grew up with two older brothers and regularly saw his grandparents and aunt. Even so, he sometimes felt alone, he said.

“You kind of feel alone because of what you went through — you lost your mother,” he said.

He, too, thanked the investigators who worked on the case.

“I was so blown away when I heard the story of how it actually transpired — how you go from what little information you have to building a case like that was truly impressive,” Byer said.

The homicide marked the fifth time sheriff’s investigators used genetic genealogy to solve a cold case. The victims in the other cases were Orbin Holloway, Teresa Solecki, Michelle Louise Wyatt and Laurie Diane Potter.

Investigators said they expect to solve more cases with the help of genetic genealogy.

Source: This post first appeared on sandiegouniontribune.com

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