The Game-Changing Technology That Solved Diane Dahn's Murder

Thirty-Four Years of Questions Finally Finds an Answer

Too many people have gotten away with murder. Think about it; there are hundreds of people who took someone's life twenty, twenty-five, thirty years ago and have spent every day since then enjoying their own. Maybe a few have been tortured by what they've done. But I've evaluated enough perpetrators to know that there are people who've done some evil things and have no trouble sleeping at night.

But that peaceful slumber may be coming to an end. As true crime buffs, you are likely familiar with genetic genealogy, a technology that uses familial DNA to determine relationships between people and find genetic matches. In the crime-fighting world, DNA technology companies use it to create family trees and, through detailed investigation, shake those branches until the perpetrator falls out. Parabon Labs has solved 140 cold cases over the past two years, the most famous of which was the Golden State Killer. Today's case is one of the most recent.

Stabbed to Death in San Diego

It was Monday, May 2, 1988, the year of the dragon. In San Diego, the temperature would reach approximately seventy-three degrees, about five above normal. Ronald Reagan was nearing the end of his second term as president. And twenty-nine-year-old Diane Lynn Dahn should have been at her job as the first female radio repair technician at the San Diego Transit Company. Except she didn't show up for her shift.

This was completely out of character for Diane. It was so unusual that a concerned coworker stopped by Diane's Santee apartment after she left work to check on her friend. She discovered Diane's nude body in her bedroom and her two-year-old son, Mark Beyer, wandering around the apartment. An autopsy determined that Diane had been stabbed and bludgeoned to death. They also found foreign DNA under her fingernails and a hair clutched in her hand. But, given the limited technology at the time, none of it was useful.

An Investigative Road Leads Nowhere

Police worked hard to find Diane's killer, but it was tough. There were none of the usual suspects; no vindictive boyfriend or ex-spouse, no fat life insurance payout waiting to be collected, no disgruntled friend or coworker, no drug or gambling debts. There was no evidence of sexual assault at the crime scene.

Diane worked hard, loved her kid, and adored her little sister. Friends described her as a free spirit with a wicked sense of humor. She loved to draw and was a talented violinist. She loved watching stock cars race at the El Cajon Speedway. Who would want to hurt Diane?

They questioned a maintenance man who had a key to the apartment, but it went nowhere. Despite their best efforts, there were never any solid leads. The lack of progress eventually pushed the case to the back burner. But not completely off the stove.

The March of Progress

Two-year-old Mark was adopted by his mother’s friend. He often visited his biological relatives and grew up with the story of his lost mom. He cherished qualities in himself that his mom's friends had described in her. And, he kept on wondering who had taken her away from him.

The San Diego County Sheriff’s office kept trying. In 2000 and 2001, as technology progressed, they tried to extract and add the DNA found underneath Diane’s fingernails to a federal DNA database from unsolved crimes. There wasn’t enough material. In 2010, a cold case team pulled out the case again to look for additional and/or missed leads and discovered the hair found in Diane’s hand. The hair DNA matched the sample under her fingernails but didn’t match any in CODIS, the federal database of convicted offenders. Diane’s killer apparently hadn’t committed any serious previous offenses, or at least he hadn’t been caught. Once again, they were at a dead end.

Fast forward to 2020, when the cold case team partnered with the crime lab and decided to give genetic genealogy a shot. This new technology allowed them to compare the DNA found at the crime scene with millions of people collected by commercially available genetic databases. If they could find relatives, they could eventually find the killer.

This work is not for the faint of heart. After identifying genealogical markers among database contributors, the team eventually constructed nine family trees with nearly 1,300 people connected to the perpetrator either through blood or marriage. Through a process of elimination, they landed on Warren Robertson.

Not Even on the Radar

There are - and will always be - unanswered questions about this case. Warren Robertson had been dead twenty-one years by the time investigators identified him. He died in a house fire on November 25, 1999 at age thirty-nine. He had been living in Indiana for the previous ten years. He had no documented history of violence, although he had been arrested for a few property crimes.

We know that, in 1988, Robertson was working as a tow truck driver and living in the same apartment complex as Diane Dahn. Like Diane, he enjoyed stock car races and often visited El Cajon Speedway. But we have no evidence that the two of them knew each other or had even met. He was never questioned by police.

Shortly after the murder, he moved away, first to Lakeside, California, and then to Indiana in 1989. We don’t know much about his life after that. We also don’t know why he targeted Diane.

Did he spot her at the races and fixate on her? Did he pass her on the apartment stairway and start stalking her? Did he plan on a sexual assault and then panic when she fought back? We will never know. But we know one thing; that he is the person responsible for her death.

The Bottom Line

I wish this story had ended with the clang of handcuffs and the promise of dying in prison. I wish Mark Beyer - once a two-year-old orphan and now a thirty-six-year-old adult - could see his mother’s killer behind bars.

But let’s focus on the big picture. After thirty-four years, we know our killer. Old cases are being solved and old offenders are being arrested. Friends, distant relatives, and strangers are coming out of the woodwork to offer saliva and blood and any other bodily fluid that can narrow down a suspect pool. Perps who have spent years feeling at ease are now learning that justice never forgets.

Finally, it feels like a turning point, a power shift toward victims and their families who were first traumatized first by violence and then by a lack of justice.

Are you as delighted about this as I am?