Too Many Heartless Murderers Get Away with It

Joni E. Johnston, Psy. D.
5 min readMar 21, 2022

After Thirty-Four Years and a Relentless Investigation, Diane Dahn’s Son Knows Who Killed His Mom

copyright free, courtesy of San Diego County Sheriff’s office

Think about this for a moment. 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 a 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…



Joni E. Johnston, Psy. D.

Forensic psychologist/private investigator//author of serial killer book. Passionate about victim’s rights, the psychology of true crime, and criminal justice.