A new HBO docuseries will reveal how Michelle McNamara was on the hunt for the Golden State Killer.

The six-part special premiering on Sunday, titled “I’ll Be Gone in the Dark,” is based on the author’s New York Times bestselling book. McNamara passed away unexpectedly in 2016 at age 46 from an accidental drug overdose.

McNamara, according to Rolling Stone, became obsessed with the Golden State Killer, a then-unknown figure who has been suspected of committing at least 12 murders, 50 rapes and 100 burglaries in California between 1974 and 1986. His other nicknames include East Area Rapist, the Original Night Stalker and the Visalia Ransacker. His trail had been cold for more than 30 years.

The armed and masked rapist would tie up any man he found in the house and pile dishes on his back, threatening to kill both victims if he heard the plates jiggle while he assaulted the women.

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According to the outlet, McNamara launched a website in 2006 titled True Crime Diary about the Golden State Killer. She went on to write articles about the case for Los Angeles Magazine. She also rode along with Paul Holes, a then-active detective who had also been hunting the Golden State Killer since the ‘90s.

“She was an investigator at heart,” Holes told Rolling Stone. “Even without any type of experience or training, she just seemed to have a knack.”

Stories of the notorious killer and his horrific acts consumed McNamara, who was determined to give a voice to his victims, the outlet noted. She later leaned on prescription drugs at times to help her sleep and concentrate especially when details of the case became too much during her writing process.

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“I really wasn’t thinking that she was having any type of trauma as a result of what she was doing on the case,” admitted Holes. “That may have been a massive oversight.”

McNamara passed away without completing her book. The manuscript was finished by fellow investigators in her honor. HBO then transformed the bestseller into a limited series directed by Liz Garbus. Her recent project was Netflix’s “Lost Girls,” a crime drama based on the real-life Long Island serial killer.

In the docuseries, viewers will learn about some of the Golden State Killer’s victims. One was 15 when she was attacked while playing the piano at home. She stopped playing it forever shortly thereafter.

“It was just a few hours, but it changed everything,” she said in the film.

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Patton Oswalt and Michelle McNamara.

Patton Oswalt and Michelle McNamara. (Photo by Gregg DeGuire/FilmMagic/Getty)

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The docuseries also focuses on how the survivors have moved forward over the years. In one scene, several are seen embracing each other after the alleged Golden State Killer is caught - which occurred just as the documentary began filming.

“It was literally the first day of filming,” said Patton Oswalt, an actor and comedian who was married to McNamara, to Rolling Stone. “We had done a book reading in Chicago that night, and we all went to bed, and we woke up the next morning and the news was out that he had been captured and arrested. And you see that in the documentary. My reaction. We’re all just like, ‘What? Where do we go from here?’”

“It’s overwhelming, but it’s hopeful,” the 51-year-old added of the legacy his late wife left behind. “It means she’s not really gone.”

DNA evidence in 2018 led authorities to arrest Joseph DeAngelo, a former police officer. While McNamara’s work didn’t directly lead investigators to the arrest, she and others dedicated to seeking answers kept pushing the case.

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Joseph James DeAngelo is believed to be the notorious Golden State Killer.

Joseph James DeAngelo is believed to be the notorious Golden State Killer. (Sacramento County Sheriff's Department)

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"I'll Be Gone in the Dark" airs June 28. The Associated Press contributed to this report.