Paul Haynes never wanted to be an amateur sleuth. He didn’t want to write a book, star in a true-crime documentary series or play a part in bringing one of America’s most prolific rapists and murderers to justice after almost 50 years. He just fell down an Internet rabbit hole and couldn’t get out again.

Haynes was 24 when he first heard about the Golden State Killer. He was studying film at university when he watched a documentary about a series of rapes in northern California. He hadn’t heard of the attacks and looked them up online. He was stunned; the same suspect was linked to at least 50 rapes and 12 murders across a decade, making him the biggest serial offender in modern history. And he was still at large. “It staggered me that this case had received so little public attention,” he says on the phone from Los Angeles. He forgot about it until a few years later, when he was working as a legal assistant.

Paul Haynes in a scene from the documentary I’ll Be Gone in the Dark.

He had a lot of downtime at work and used public records to find men who fit the criteria of this offender. The man known variously as the Original Night Stalker, the East Area Rapist and, ultimately, the Golden State Killer had a particular style. He would watch victims for days before he struck, unlocking windows and leaving back gates open to prepare. He wore gloves and a mask and attacked only at night. At first he stuck to single victims, but he soon moved on to couples. Husbands were tied up and had plates placed on their back. “If I hear the plates rattle, I’ll kill your wife,” they were told. Even children were no barrier — one of his victims recalled her three-year-old son being sent to his bedroom before she was raped.

Haynes knew his research was an odd thing to do, so he kept it under wraps. But the following year his law firm closed and Haynes moved to south Florida to live with his “highly dysfunctional family”, feeling deeply demoralized. “Instead of marinating in my depression, I instinctively drifted into doing this more or less full time,” he says. There was a community of people like him sharing information online. Some were retired police officers. Some were victims themselves. The random nature of the attacks made this the most fascinating kind of case, he says: “You have a thousand different haystacks and only one needle.”

The man known variously as the Original Night Stalker, the East Area Rapist and, ultimately, the Golden State Killer had a particular style.

Haynes, though, is a data man. He started digitizing telephone directories, compiling master lists based on marriage and birth records and data mined for anyone who was in the right place at the right time. He began posting on cold case forums and it was here that he met Michelle McNamara. She was the journalist behind True Crime Diary, a popular blog, and the wife of Patton Oswalt, a well-known comedian in America. She too was fascinated by the case and they started swapping details.

McNamara was juggling her sleuthing with a busy family life. She worked at night from her daughter’s playroom. They teamed up to research an article for Los Angeles magazine that led to a book deal, then Haynes moved to LA to work with her full time.

One of the biggest problems was that the attacker had moved across the state as his crimes got more serious. The rapes took place mostly in Sacramento, but as he progressed to murder he went south to Santa Barbara. There was also a series of earlier burglaries in Visalia that bore many similar trademarks — the perpetrator watched his victims at home and masturbated. Lack of cooperation between police in different jurisdictions had stalled the manhunt, as had the culture of victim-blaming rife at the time.

Writer Michelle McNamara in I’ll Be Gone in the Dark.

McNamara had a gift for wooing stubborn police officials and winning the trust of victims and their families. It was “dark, frightening material”, Haynes recalls. Six out of the first 10 victims were teenagers. One of them, Kris Pedretti, had skipped a high-school dance because she had a cold and was at home alone playing the piano when he attacked her. He held a knife to her neck and said: “If you make a noise I will put this through your throat and I’ll be gone in the dark.” He then raped her three times.

Husbands were tied up and had plates placed on their back. “If I hear the plates rattle, I’ll kill your wife,” they were told.

Sometimes he called his victims afterward. McNamara got hold of a police recording. “I’m going to kill you,” a rasping voice says over and over again. McNamara was consumed in her work, Haynes says. The book had given them legitimacy, but it took a back seat from her real goal: tracking down the Golden State Killer. She had herself been raped and Haynes says she had a particular interest in cases where victims were attacked while asleep. Haynes often wonders why he too is fascinated by something so grim. “I suffered extraordinary abuse in the past,” he says. “Whenever I wash my face in the sink, I have this paranoia that when I open my eyes there is going to be a person behind me.”

The Golden State Killer tapped into a primal fear. “He targeted couples in upper-middle-class neighborhoods in locked homes in the middle of the night. You can’t imagine yourself not being vulnerable to an offender like this.”

McNamara had procured 37 boxes of police records from the Orange County sheriff’s department. She and Haynes trawled through them for leads, but she always felt advances in DNA science were their best hope.

She had also got hold of the offender DNA markers from one of the police forces involved. Some ancestry websites allowed you to upload genetic material, but the results were vague. “It would spit back surnames of interest, places of origin,” Haynes recalls. Certainly McNamara was excited about the progress they were making. But on April 21, 2016, everything came to a stop — McNamara died in her sleep.

It was “dark, frightening material”, Haynes recalls. Six out of the first 10 victims were teenagers.

The cause of death was accidental overdose; she had been taking prescription drugs, fentanyl and other opiates to help her sleep. Haynes was stunned. He has raked over their time working together for clues that his colleague was struggling — and found none.

“She would tell me she thought there was an intruder one night, that she swung the bedside lamp at Patton,” he says. “She might have mentioned a nightmare or two, but I guess it didn’t register.”

Oswalt and Haynes joined efforts with Billy Jensen, another crime author, to finish her book. I’ll Be Gone in the Dark went to No 1 on the New York Times bestseller list and was snapped up for a documentary by HBO, but the killer was still at large. That changed in April 2018, when Joseph James DeAngelo Jr, a 72-year-old former policeman, was arrested under suspicion of being the Golden State Killer.

That night Haynes was in Chicago with Oswalt promoting the book. “It was like a punch in the gut,” he says, “a sudden surge of sadness because Michelle was not there to be rewarded with the thing she had sought.”

McNamara had been right — it was DNA that eventually led the police to DeAngelo’s door. A website called GEDmatch allowed the public to upload their DNA and trace family members from the database. The police did this with samples of the attacker’s DNA; they found some of DeAngelo’s distant cousins and plotted a reverse family tree. DeAngelo had worked as a police officer from 1973 to 1976 in Exeter, a city between Sacramento and LA close to Visalia. He then joined the Auburn police department, northeast of Sacramento. The murders stopped the year his first daughter was born.

DeAngelo was not on the list of names that he and McNamara had drawn up. “It was not the face and name I had imagined,” Haynes says, but public interest in the case had soared as a result of McNamara’s involvement. So too had the possibilities of DNA technology. This was the first high-profile case solved using “reverse genealogy”, and about 120 cases have since been solved in this way. The world of amateur sleuthing is abuzz with the potential.

Last year Haynes went to CrimeCon, a convention for true-crime enthusiasts, to discuss the case. It was “weirdly upbeat and celebratory”, he says. He still has a lot of anxiety about being characterized as a citizen detective: “It has a lot of negativity. It smacks of marginality.” He hopes “to exit this story altogether at some point soon.”

I’ll Be Gone in the Dark is on Now TV, Sky Crime beginning August 30, and currently on HBO in the U.S.