On May 2, 1988, twenty-nine-year-old Diane Don did not show up for her shift at the San Diego Transit Corporation, where she worked as a radio repair technician. When she failed to answer her telephone, a co-worker named Gary Beyer drove to her apartment complex in Santee, California, to check on her. He found the front door open. Inside, in the front room, Diane’s two-year-old son Mark was walking around alone.

Beyer went into the bedroom and found Diane dead. The call Beyer made to emergency dispatchers still exists. There is nothing dramatic in it, only the voice of a man struggling to process what he had walked into. “There is a lady laying on the ground,” he says.

“She’s gone. ” Within minutes, deputies from the San Diego County Sheriff’s Department were on the scene. The evidence they collected told a specific story. There was no sign of forced entry.

Whatever had happened had happened quickly. Detectives lifted fingerprints, gathered clothing, and took scrapings from beneath Diane’s fingernails. They also collected hairs from the apartment, and someone had the presence of mind to note that one of those hairs had come out of her hand. That detail would matter more than anyone in that room could possibly have known.

Diane had fought her attacker. The scrapings from under her nails carried a male profile that was not hers. The hair in her hand was not her own. In her final moments, she had reached up, grabbed hold of the man who was assaulting her, and pulled.

She tore a strand of hair from his head, root and all, and she kept her fist closed until she died. Whoever came into that apartment did not walk out of it untouched. There was one other witness. Mark was physically unharmed, but he was too young to describe what he had seen.

With a psychologist present, detectives gently questioned the child. He gave them fragments: the shape of an adult he had seen, and a statement that a bad man hurt Mommy. That was the whole of the eyewitness account, and Mark grew up with no memory of giving it. Decades later, as a grown man, he would say that the question that stayed with him was not what had happened to his mother, but why he had survived.

“There’s a certain amount of questions that come along with that,” he said. “Like why? Why didn’t I die? ”

Diane was born in New Jersey in 1959 and grew up in the eastern part of San Diego County after her family moved west.

She had spent time in Austria as a foreign exchange student. She played the violin and the guitar, and she was genuinely good. She drew and had a sharp sense of humor. Her younger sister Victoria was close to her, and by 1986, both were mothers of small children.

Diane was a fun-loving free spirit, according to those who knew her. At the Transit Corporation, she was the first woman to hold her job, repairing the radios that kept the city’s transit system talking to itself. She also loved stock car racing and went to watch at the El Cajon Speedway. The evening before she died, Diane and Mark had dinner with Gary Beyer and his wife Marilyn.

When the question arose of what would happen to a two-year-old boy who no longer had a mother, Gary and Marilyn stepped forward. Mark grew up as their son and took their name. “I was loved, I had a good life,” he said later. “I had all the things that a normal child would have.

”

For the next two decades, the investigation went nowhere. Detectives canvassed the apartment complex and questioned neighbors, boyfriends, co-workers, and a maintenance man who had a key. Every name they developed fell away. There was no estranged husband, no jealous ex, no debt, no threat anyone could recall.

Nobody who knew Diane could think of a single person who would want to hurt her. The case went quiet and went into the archives. But it was never abandoned. In October 2000, twelve years after the murder, the Sheriff’s Department returned to the fingernail scrapings and pulled a DNA profile.

It led to nobody. A second attempt in 2001 produced a profile of insufficient quality to be entered into CODIS, the national database of DNA from convicted offenders. In 2010, a cold case team pulled the file again and turned to the hairs. A criminologist named Kathy Chang went through more than two hundred of them.

Most matched Diane, which was expected. But one did not. One hair carried the profile of an unknown male. It did not match Diane, and it did not match any of the reference samples from people who had been cleared over the previous twenty-two years.

That hair had been torn from someone’s head while the root was still alive, and it still contained enough material to build a profile. The profile was entered into CODIS, and nothing happened. No hit came back. CODIS holds profiles from convicted offenders, but if the man was never convicted of anything serious, he would never be in it.

They had his genetic profile, but they could not put a name to it. What changed everything happened four hundred miles up the coast. In April 2018, investigators in Sacramento announced they had identified the Golden State Killer using a new technique. They had taken crime scene DNA, processed it into the kind of file consumer ancestry services use, uploaded it to a public genealogy database, and looked not for the man himself but for his relatives.

They built family trees until they arrived at one person who fit. The method is called investigative genetic genealogy. Its logic is simple: you do not have to be in a database to be findable. You only need your cousins to be.

Most agencies using this technique send the profile to a commercial company that builds the trees. San Diego decided to learn to do it themselves. The advantage was the case file. A commercial lab builds a tree exhaustively, but a detective with the file open beside them could look at a branch and ask which of these people has any connection to San Diego.

In Diane Don’s case, that turned out to be the whole ballgame. In May 2020, thirty-two years after the murder, the cold case team formally began working Diane’s case using investigative genetic genealogy. Chang still had enough DNA left from the hair to send out for the kind of analysis genealogy depends on. The profile was uploaded to GEDmatch, a database that accepts files from consumer testing services.

The highest initial match came back at almost two hundred centimorgans, roughly a second cousin. It meant nobody would have to travel back eight or nine generations before coming forward again. Then the real labor began. They built outward and backward from that first match, and at one point the search reached back to a common ancestor born in 1817.

Nine separate family trees were constructed, containing nearly thirteen hundred individuals connected to one unknown man by descent or by marriage. Thirteen hundred people, almost none of whom had ever heard of Diane Don. The first promising branch contained people with deep San Diego connections and felony records, but they had nothing to do with what happened to Diane. Most of what the team did across eighteen months was eliminate people.

Every failure was progress, because it narrowed what remained. Then they worked a different side of the tree, and there they found another cluster of people with a relationship to San Diego. A couple of them had a relationship to Santee. That was the moment the case turned.

They narrowed further, looking at where each candidate had lived in the 1980s, and eventually they were left with one man. His name was Warren Robertson. Robertson was born in Arkansas and had spent most of his younger life in the San Diego region. In 1988, he was about twenty-seven years old, almost exactly Diane’s age.

He worked as a tow truck driver. He was married and had a son. And in 1988, he lived in the same apartment complex on Graves Avenue, in a building just off Diane’s, positioned so that he could see her. He had never been a suspect.

He had never been interviewed. In the entire original file, his name did not appear once. The only overlap anyone ever established between the two of them was the racetrack. Diane went to the El Cajon Speedway to watch the stock cars.

So did Warren Robertson in the year she died. Whether they ever met there, nobody knows. “Maybe that’s where they met,” one detective said. “It’s hard to tell.

”

Robertson’s record explained why he had never turned up in CODIS. He had been arrested for low-level offenses only, drug offenses and property crimes, nothing violent, nothing that required a DNA sample. Soon after Diane died, he left his wife and son and moved out of Santee, and in late 1989 he left California for Indiana. On November 25, 1999, Warren Robertson died in a house fire.

He was thirty-nine years old, twenty-one years before anyone attached his name to Diane Don’s case. There would be no arrest, no interview, no trial, and no opportunity for anyone to ask him why. But a family tree is a hypothesis. They still needed DNA.

They identified Robertson’s son, and they called him. They told him an extraordinary story about a man he had barely known, and they asked if he would give a sample. He said yes. “I don’t want to think of my father as somebody who took a life,” he told them.

The crime lab ran a paternity-type comparison using the son’s DNA and the son’s mother’s DNA, asking one question: could the man who left that hair in Diane Don’s hand be this person’s father? The results came back with an extraordinarily high probability that the answer was yes. One of the first people they told was Mark. He was driving to the coast when his phone showed a text message from the detective.

He called back and could not take it in. “I was in shock,” he said. Later, the detective walked him through the genealogy, the trees, the relatives, the grind of it. “I thought, my god, you’ve got to be kidding me,” Mark said.

“Like all of that stuff is now possible. It just blew my mind. ”

On March 16, 2022, the San Diego County Sheriff’s Department went public. Mark did not attend in person.

He sent a recorded video statement. His aunt Victoria, Diane’s younger sister, was there in the room. She had spent thirty-four years telephoning detectives to ask whether anything had changed. She did not get through the announcement without breaking down.

“My sister was an amazing person,” she said. “She loved life and always played the violin and guitar. ” She spoke about what the answer meant to her. “It probably was, other than the birth of my children, the best day of my life,” she said.

“It doesn’t make the pain go away, but at least there’s an answer. ”

Mark’s recorded statement was shorter. What came through was not anger at the man who had been named. It was a plain description of what it is to grow up on the far side of something like this.

Then he turned to the detectives, and what he wanted on the record was gratitude. “I was so blown away when I heard the story of how it actually transpired,” he said. “The answers that my family received, it’s closure, and closure is everything, even after so much time had passed. ”

The announcement came just weeks after what would have been Diane’s sixty-third birthday.

It was the fifth cold case the sheriff’s homicide unit had resolved using investigative genetic genealogy. Investigators stated without hedging that the investigation revealed substantial and convincing evidence that Robertson was responsible for Diane’s death. What the case does not have is the part everyone wants. There was no arrest, no conviction, no courtroom, and no motive, and there never will be one.

Everything in this story runs through that envelope in the evidence room. Not the canvass of the apartment complex, which found nobody. Not the fingerprints. Not the interviews with boyfriends and co-workers.

Not a two-year-old’s fragmentary description of an adult he had seen. What broke the case was one strand of hair that Diane Don tore out of the man attacking her in the last minutes of her life, a hair that came away with living tissue at the root because of the force with which she pulled, and that stayed closed inside her hand until strangers arrived to open it. She could not tell anyone his name, because she never knew it. So she took a piece of him instead, and thirty-two years later a criminologist found it among two hundred others, and it turned out to be the only thing in the entire file that was ever going to lead anywhere.

Diane identified the man responsible for her own death. It simply took the rest of the world thirty-four years to catch up with what she had already done. Her son is a grown man now. He has her photographs and her old marine biology notes, with her handwriting in the margins about creatures he went on to love.

When he wants to be near her, he drives out to the coast in Oregon, hundreds of miles up from San Diego, and stands at the edge of the water where her ashes went. Different beach, same ocean. “I feel connected,” he said. “I feel close.

And it’s almost as if you can just feel her presence. ”

She never got to raise him. She never got to hear anyone tell her he was just like her. But she fought for him in that bedroom in 1988, and she held on to the one thing that would eventually give him an answer.

She did not let go.