Three days before she died, Michelle McNamara typed some notes in a cryptic to-do list on her laptop.

Among the looming tasks: “Find out from Debbi D about flashlight.” “Find out from Ken exactly what he meant about the husband or the guy in the clown suit walking down the street.”

The list cataloged potential clues she planned to chase down as she completed her book, “I’ll Be Gone in the Dark,” an exhaustive investigation into the identity of the Golden State Killer, who committed upward of 50 sexual assaults and at least 10 murders in California in the 1970s and 1980s.

She never finished it.

On April 21, 2016, her husband, comedian Patton Oswalt, found her dead in the bedroom of their Los Angeles home. An autopsy showed that McNamara, who was 46, had an undiagnosed heart condition, and had taken a dangerous mix of prescription drugs, including Adderall, the pain narcotic fentanyl and the anti-anxiety medication Xanax.

The story that she spent the last five years of her life obsessively researching was half written, the gruesome mystery still unsolved. After losing his wife, Oswalt couldn’t bear the thought of her work languishing.

“This book had to be finished,” he said in a telephone interview. “Knowing how horrible this guy was, there was this feeling of, you’re not going to silence another victim. Michelle died, but her testimony is going to get out there.”

Shortly after her death, Oswalt recruited Billy Jensen, an investigative journalist, and Paul Haynes, who worked closely with McNamara on the book as a researcher, to comb through her handwritten notes and the roughly 3,500 files on her computer and piece together the story she set out to tell.

“I’ll Be Gone in the Dark,” due out Feb. 27, is a vivid and meticulous investigation of a twisted predator who terrorized quiet, upper middle-class communities in California for nearly a decade, and a wrenching personal account from a writer who became consumed by her subject. It has drawn accolades from some of the country’s top crime and horror writers, including Stephen King, Michael Connelly, Megan Abbott and Gillian Flynn, who wrote an introduction to the book.

The tragedy of McNamara’s death became a meta-narrative running through the book. Its pages are punctuated with recurring editor’s notes that serve as a reminder that the author, who is so palpably present on the page, is absent from the world. She wrote frankly about the psychological toll the project took on her, how immersing herself in the grisly details left her emotionally frayed.

“There’s a scream permanently lodged in my throat now,” McNamara wrote.

Rather than attempting to mimic her voice and flesh out fragmentary chapters, or condense her sprawling research into a taut true crime narrative, Haynes and Jensen let the jagged edges of the unfinished project show. They preserved her completed chapters, which recount the killer’s attacks in unsparing detail and examine his methodology, and explore her own fascination with unsolved crimes and her evolution as an amateur detective. Other chapters were pieced together from her notes, and are marked with disclaimers. Some sections read like raw, unfiltered research: one mesmerizing chapter consists entirely of a transcript from McNamara’s interview with Paul Holes, a criminalist in the Contra Costa Sheriff’s Office.

Toward the end of the book, just as McNamara’s investigation seems to be gaining momentum and the killer’s hazy profile begins to come into focus, it grinds to a halt. An editor’s note explains how, after McNamara died, Haynes and Jensen were brought in to “tie up loose ends and organize the materials Michelle left behind.” In that final section, Haynes and Jensen lay out some of the avenues McNamara had planned to explore. They describe what they found on her laptop — old maps and aerial photographs of Goleta, the site of multiple murders, images of shoe prints found at crime scenes, and a spreadsheet with names and addresses of men who competed on a 1976 high school cross-country team (she thought the perpetrator might be a runner, based on victims’ descriptions of his muscular legs). They explore the potential for using DNA and genealogy databases to identify the killer’s family, which McNamara believed was the best route for finding a criminal who had evaded investigators for four decades.

It took Haynes and Jensen about a year to put the book together, and one can feel their frustration at wrestling with all of the evidence and theories McNamara compiled, and still coming up empty.

Haynes, who worked closely with McNamara for several years, was stunned and devastated by her death, he said. He also had to wrestle with the sweeping territory McNamara intended to cover.

“It’s a very ambitious book to write, about a case like this with a scope as vast as it is,” Haynes said. “The question was, what holes should we attempt to patch?”

McNamara became fascinated with unsolved crimes when she was growing up in Oak Park, Illinois, the youngest of six siblings in a large Irish Catholic family. When McNamara was 14, a young woman named Kathleen Lombardo was murdered near the McNamaras’ home. More curious than afraid, McNamara went to the alley where the body was found, and picked up shards of the victim’s broken Walkman. The killer was never caught.

She was living in Los Angeles and writing screenplays and TV pilots when she met Oswalt in 2003, at one of his comedy shows. They went on a few dates and bonded over their shared obsession with serial killers. They got married a couple of years later, and Oswalt urged her to channel her grim hobby into writing. In 2006, she launched her website, True Crime Diary, where she chronicled hundreds of unsolved crimes.

In 2011, she wrote on her blog about a string of unsolved rapes and murders from the 1970s and 1980s that were committed by an unidentified man who was known as the East Area Rapist and the Original Night Stalker.

“I’m obsessed,” she wrote. “It’s not healthy.”

For a true crime addict, the case was tantalizingly complex. So much was known about his methods and even his psychology, yet the killer had thwarted investigators for decades. He was a meticulous planner who stalked his targets in advance, learning their daily routines before breaking into their homes. He brought his own precut ligatures to tie victims up, and always wore a mask. He stole objects that had sentimental value for the victims, like engraved jewelry and class rings. He grew increasingly confident, and went from assaulting women who were home alone to attacking couples in their bedrooms. She gave him a catchier name: the Golden State Killer.

McNamara wrote about the case for Los Angeles magazine, and signed a book deal with Harper several months later.

The research consumed her, and began to weigh on her. She suffered from insomnia and anxiety. Once, she panicked because she woke up to a scraping sound: A neighbor was dragging his trash can to the curb in the middle of the night, Oswalt said. Another time, when Oswalt tiptoed into their bedroom, trying not to wake her, she mistook him for an intruder and jumped out of bed and swung a lamp at his head. She felt an obligation to solve the case, and was devastated each time she developed a promising theory or zeroed in on a suspect but failed to find sufficient evidence.

“She had overloaded her mind with information with very dark implications,” Oswalt said.

Oswalt wasn’t aware of all the prescriptions she was taking or what the medications were for, he said. Both of them led busy work lives and were devoted parents to their then 7-year-old daughter, Alice, and McNamara seemed to be managing the stress. It wasn’t until he saw the coroner’s report months after her death that he realized that McNamara was coping, in part, by taking prescription drugs.

“It’s so clear that the stress led her to make some bad choices in terms of the pharmaceuticals she was using,” he said. “She just took this stuff on, and she didn’t have the years of being a hardened detective to compartmentalize it.”

Oswalt, who married actress Meredith Salenger last year, said he thinks there’s a chance the killer will eventually be caught, due, in part, to the work McNamara did and the attention it brought to a decades old cold case.

McNamara believed that, too. In a letter to the killer that appears at the end of “I’ll Be Gone in the Dark,” she addresses him directly, and says it’s only a matter of time until officers arrive at his door.

“This is how it ends for you,” she writes.