The San Diego Sheriff’s Department has solved the cold-case murder of Diane Dahn, who was 29 years old when she was stabbed to death in her apartment in 1988.
Genealogical DNA testing linked Warren Robertson to the crime. Robertson died in a house fire in Indiana in 1999, according to authorities.
“The investigation revealed substantial and convincing evidence that Robertson had murdered Diane,” the Sheriff’s Department said.
Police said they didn’t know if the two knew each other but the suspect and the victim both attended car races at El Cajon Speedway.
According to investigators, they used Dahn’s fingernail scrapings and hair for DNA testing.
Police tested Dahn’s DNA in 2000 and 2001 but didn’t find a match. The Sheriff’s Office Cold Case Team retested the DNA in 2010 using new technology. However, it still did not yield a suspect from a federal database of DNA for unsolved crimes.
The Cold Case Team began using genealogical testing in May 2020. Officials connected Robertson to the case following 18 months of testing.
Officials say it was the fifth murder solved with this testing method, which uses commercial DNA databases to find possible relatives of suspects.
The team formed “family trees,” which led authorities to other potential relatives.
“This process was so exhaustive, nine family trees were constructed with nearly 1,300 people connected to Robertson either through blood or marriage,” the sheriff’s office said. “This murder would likely have gone unsolved if not for the use of investigative genetic genealogy.”
Dahn’s son Mark Beyer — who was two years old when his mother was killed in their Santee, California, apartment — was shocked by how detectives cracked the case.
“Looking back, some of the struggles you go through is you feel alone because of what you went through. You lost your mother,” Mr. Beyer said in a video statement. “The answers that my family received is closure, and closure is everything even after so much time had passed.”
• Peter Santo can be reached at psanto@washingtontimes.com.
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