Ms. McNamara became fascinated with unsolved crimes when she was growing up in Oak Park, Ill., the youngest of six siblings in a large Irish Catholic family. When Ms. McNamara was 14, a young woman named Kathleen Lombardo was murdered near the McNamaras’s home. More curious than afraid, Ms. 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 Mr. 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 Mr. 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.