At night, after tucking her daughter into bed, Michelle McNamara brushed her teeth, put on her pyjamas and got ready for part two of her double life.

During the day she was a doting mother and wife but at night she tracked one of America's most prolific — and elusive — serial killers.

Behind her 15-inch laptop in downtown Los Angeles, McNamara spent countless hours doing the work police didn't have time to do. She refused to let a cold case stay cold forever.

McNamara pored over autopsy reports and crime scene photographs and 1970s-era police files chasing a man who carried out 12 murders and almost 50 rapes over two decades before disappearing without a trace in 1986.

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Joseph James DeAngelo is thought to be the Golden State Killer. Photo / news.com.au
Joseph James DeAngelo is thought to be the Golden State Killer. Photo / news.com.au

She even coined the killer's catchy nickname, disregarding the monikers bestowed on the suspect by police in the many jurisdictions where he struck, in favour of a title that sewed the state's geography together: the Golden State Killer.

McNamara published her findings on the True Crime Diary website and later wrote them down in more detail in the book I'll Be Gone In The Dark, published posthumously last month, news.com.au reports.

On Wednesday, police believe they got their man — a 72-year-old Vietnam War veteran and former police officer named Joseph James DeAngelo. If McNamara didn't lead them to his tidy, suburban doorstep, she certainly helped.

Sadly, the author isn't around to see the killer caught. She died, aged 46, in her sleep in April 2016. Her husband, comedian and actor Patton Oswalt, made it his mission to get his late wife's book published.

It made The New York Times' bestseller list and, on Thursday, Oswalt wrote on Twitter: "I hope you got him, Michelle. I hope they got him."

Finding a needle in a haystack

DeAngelo maintained a neat lawn and mostly kept to himself inside his Citrus Heights home where police came knocking almost 40 years after he allegedly began his crime spree.

Police say they finally have the man known around the United States as the Golden State Killer, the East Area Rapist and the Original Stalker.

Sacramento County Sheriffs deputies search the California home of Joseph James DeAngelo. Photo / AP
Sacramento County Sheriffs deputies search the California home of Joseph James DeAngelo. Photo / AP

Police say he entered homes up and down the coast armed with a gun and wearing a balaclava while single women or couples were sleeping. The killer sometimes tied up the man and piled dishes on his back, then raped the woman while threatening to kill them both if the dishes tumbled.

He often took souvenirs, notably coins and jewellery, from his victims, who ranged in age from 13 to 41.

Police used DNA to finally track down DeAngelo and tie him to the sadistic crime spree but refused to give specifics about how it was collected or matched to the suspect.

They admitted that despite decades of work, DeAngelo's name had not been on authorities' radar before last week.

"We knew we were looking for a needle in a haystack, but we also knew that needle was there," District Attorney Anne Marie Schubert said.

"We found the needle in the haystack ... the answer was always going to be in the DNA."

Joseph James DeAngelo has been arrested on suspicion of at least 12 killings and 45 rapes. Photo / AP
Joseph James DeAngelo has been arrested on suspicion of at least 12 killings and 45 rapes. Photo / AP

DeAngelo was fired from the Auburn Police Department, two hours north of Sacramento, in 1979 after he was arrested for stealing a can of dog repellent and a hammer from a drugstore.

Jane Carson-Sandler, who was sexually assaulted in California in 1976 by a man believed to be the so-called "East Area Rapist", said she received an email on Wednesday from a retired detective who worked on the case telling her they have identified the suspected rapist and he's in custody.

"I have just been overjoyed, ecstatic. It's an emotional rollercoaster right now," Carson-Sandler, who now lives near Hilton Head, South Carolina, told The Associated Press in a telephone interview.

"I feel like I'm in the middle of a dream and I'm going to wake up and it's not going to be true. It's just so nice to have closure and to know he's in jail."

'The flashlight's blaze forced open their eyes'

In her book, McNamara describes in detail the methods the Golden State Killer used when preying on his victims. She says he was meticulous: disabling porch lights and entering through unlocked sliding doors.

"Precision and self-preservation were his identifying features," she wrote. "When he zeroed in on a victim, he often entered the home beforehand when no one was there, studying family pictures, learning the layout.

A house ransacked after a home invasion by the East Area Rapist. Photo / AP
A house ransacked after a home invasion by the East Area Rapist. Photo / AP

"He emptied bullets from guns. Unworried homeowners' closed gates were left open; pictures he moved were put back, chalked up to the disorder of daily life. The victims slept untroubled until the flashlight's blaze forced open their eyes. Blindness disoriented them. Sleepy minds lumbered, then raced. A figure they couldn't see wielded the light, but who, and why?

"Their fear found direction when they heard the voice, described as a guttural whisper through clenched teeth, abrupt and threatening, though some detected an occasional lapse into a higher pitch, a tremble, a stutter, as if the masked stranger in the dark was hiding not only his face but also a raw unsteadiness he couldn't always disguise."

For her, chasing the Golden State Killer wasn't just a hobby. Unsolved murders were something she'd been interested in since she was a teenager when a neighbour was murdered.

She had everything she needed in that laptop once public records came online along with sophisticated search engines.

"I recognised how a head full of crime details could intersect with an empty search bar," she wrote.

"When my family goes to sleep, I time travel and reframe stale evidence using 21st-century technology. I start clicking, scouring the internet for digital clues authorities may have overlooked, combing digitised phone books, yearbooks and Google Earth views of crime scenes — a bottomless pit of potential leads for the laptop investigator who now exists in the virtual world."

In her book, Michelle McNamara (pictured here with husband Patton Oswalt) describes in detail the methods the Golden State Killer used when preying on his victims. Photo / AP
In her book, Michelle McNamara (pictured here with husband Patton Oswalt) describes in detail the methods the Golden State Killer used when preying on his victims. Photo / AP

Police on Thursday acknowledged McNamara's body of work but stopped short of saying she led them to DeAngelo.

"It kept interest and tips coming in," Sacramento County Sheriff Scott Jones said.

Oswald responded on Twitter: "The cops will NEVER and HAVE NEVER credited a writer or journalist for helping them solve a case."

He said his wife "didn't care about getting any shine on herself ... she cared about the Golden State Killer being behind bars and the victims getting some relief".

A dogged person fixated on the case

McNamara's long fascination with true-crime stories sprouted from an unsolved murder near her family's home in Oak Park when she was 14.

After moving to Los Angeles later, she worked briefly for a private detective, before going on to write for television, Sarah Stanard, a longtime friend of McNamara's told The Washington Post. But she returned to focus on her fascination with true crime, starting a blog in 2006 to examine unsolved cases.

"She was just a dogged person," Stanard said. "She had a different brain."

A sketch of a stolen ring the East Area Rapist took from one of his victims. Photo / AP
A sketch of a stolen ring the East Area Rapist took from one of his victims. Photo / AP

One of the cases she examined was the unknown assailant known variously as the East Area Rapist, for early crimes committed in Sacramento, the Original Night Stalker, after police learned that his crimes predated that of serial killer Richard Ramirez, and the Visalia Ransacker.

The killer left a lurid trail of crime scenes and victims across the large state: horrifying home invasions, women raped in front of their bound loved ones, and a series of couples killed in their homes together. His meticulousness helped him elude capture for decades.

"Victims received hang-up or disturbing phone calls before and after they were attacked," McNamara wrote. "He disabled porch lights and unlocked windows. He emptied bullets from guns. He hid shoelaces or rope under cushions to use as ligatures. These manoeuvres gave him a crucial advantage because when you woke from a deep sleep to the blinding flashlight and ski-masked presence, he was always a stranger to you, but you were not to him."

For her, the case had become a fixation. She joined with other amateur sleuths in online message boards, met with survivors of the killer's victims, pored over decades-old files, autopsy reports, maps and mug shots.

Sacramento County Sheriff Scott Jones discusses the arrest of Joseph James DeAngelo for a string of violent crimes. Photo / AP
Sacramento County Sheriff Scott Jones discusses the arrest of Joseph James DeAngelo for a string of violent crimes. Photo / AP

"I'm obsessed," she wrote on her blog in 2011. "It's not healthy."

She wrote a long narrative about the case and her obsession with it for Los Angeles Magazine in 2013, which led to a book deal with HarperCollins.

"And, you know, in writing the book, she began to recruit retired homicide detectives and cops from all these different jurisdictions and precincts and cities. And she got them to pool information," Oswalt said last year during an interview on NPR's Fresh Air.

"But her research was so meticulous and so complete that they would contact each other and say, talk to Michelle. She knows. This person is actually not some weird, you know, overenthusiastic amateur. She wants to put the bracelets on this guy."

Investigation takes its toll

But the work began to take a toll. McNamara, who would often work at night after her daughter and husband went to bed, began to develop anxiety and sleep issues, Oswalt said. He has spoken about the panic the case created for her, including one time she mistook him for an intruder in the middle of the night and swung a lamp at his head.

"I think that is what led her down this road of using Xanax. And I know she was taking Adderall in the mornings to get up and some — you know, before she died, the three days before she died, she really didn't sleep because there was all this new breaking stuff on the case," Oswalt said. "I'm not going to be glib and say that's the cause of death. The cause of death was a lot of things. But that certainly held the door open for the other causes."

She died in her sleep from what host Terry Gross said was an undiagnosed heart problem along with Xanax, Adderall and fentanyl in her system on April 21, 2016. She was 46.

Part of the East Area Rapist Crime reports at the Sheriff's department evidence room in Sacramento. Photo / AP
Part of the East Area Rapist Crime reports at the Sheriff's department evidence room in Sacramento. Photo / AP

Oswalt helped steward the book's completion, with the help of a journalist, Billy Jensen, and a researcher, Paul Haynes. It has been hailed by critics, writers such as Stephen King and Gillian Flynn, and readers, and landed on bestseller lists.

Family and friends said they had just done a reading of her book at a Chicago-area bookstore around the time they believe DeAngelo was arrested.

"On the night when all of Michelle's collaborators were together for the first time, in Michelle's hometown, with Michelle's family present, the monster we sought is simultaneously taken into custody," researcher Paul Haynes wrote. "I'm a rational man, but I can't help but feel this transcends coincidence."

Many shared a particularly prescient passage from the book that envisioned the suspect's arrest in the future.

"One day soon, you'll hear a car pull up to your curb, an engine cut out. You'll hear footsteps coming up your front walk," McNamara wrote. "The doorbell rings. No side gates are left open. You're long past leaping over a fence. Take one of your hyper, gulping breaths. Clench your teeth. Inch timidly toward the insistent bell. This is how it ends for you."

— Washington Post, news.com.au, AP