34 Years After Murder, Killer Identified From a Single Hair

'Closure is everything,' says son who was 2 when Diane Dahn was killed
By Rob Quinn,  Newser Staff
Posted Mar 18, 2022 10:28 AM CDT
34 Years After Murder, Killer Identified From a Single Hair
The sheriff's department says the investigation "revealed substantial and convincing evidence that Robertson had murdered Diane."   (San Diego County Sheriff's Department)

(Newser) – On May 2, 1988, 2-year-old Mark Beyer was found wandering on his own in a San Diego-area apartment complex. His mother had been stabbed to death in their apartment. After 34 years, police have identified the killer, and Beyer, now 36, says he finally has closure—"and closure is everything, even after so much time had passed." Authorities used forensic genealogy to identify Diane Dahn's killer as Warren Robertson, a tow truck driver who lived in the same complex, CBS reports. Robertson, who was never identified as a suspect in the initial investigation, died in a house fire in Indiana in 1999 at age 39.

Beyer said in a video statement said he was "blown away" by how investigators had managed to find the killer using DNA from a hair follicle found in his mother's hand. In a process crime intelligence analyst Jeffrey Vandersip describes as "extremely laborious," investigators found matches on genealogy websites and constructed nine family trees with nearly 1,300 people connected to Robertson. The sheriff's office says direct relatives helped the investigation by providing DNA samples. The motive is still unknown. Investigators say Dahn, 29, may have known Robertson from a shared interest in stock car races, the San Diego Union-Tribune reports.

Dahn was not sexually assaulted and Robertson, who left his family soon after the murder, had a record of low-level crimes against property but no history of violent crimes. Beyer was adopted by a family friend and continued to see his grandparents and aunt, but he says he still often felt alone, NBC San Diego reports. "She was a good person. Everything I’ve ever been told and heard about her is that she was a good person, and I like to think that those values have transferred off to me and I do my best to be a good person," he says. "The world could always use more good people." (Read more cold cases stories.)

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