MICHELLE McNamara lived a double life. By day she worked in television, and was the wife of comedian and actor Patton Oswalt. At night she ran a true crime website, diving deep into unsolved crimes. One in particular grabbed her — the East Area Rapist.

The following is an extract from her book I’ll Be Gone In The Dark, about her obsession with one man, and her determination to catch someone who had escaped authorities for decades. Sadly, McNamara died in 2016 with this book unfinished. With the help of Gillian Flynn and McNamara’s husband, the book has now been published.

THAT summer, I hunted the serial killer at night from my daughter’s playroom.

For the most part I mimicked the bedtime routine of a normal person. Teeth brushed. Pyjamas on. But after my husband and daughter fell asleep, I’d retreat to my makeshift workspace and boot up my laptop, that 15-inch-wide hatch of endless possibilities.

Our neighbourhood northwest of downtown Los Angeles is remarkably quiet at night. Sometimes the only sound was the click as I tapped ever closer down the driveways of men I didn’t know using Google Street View. I rarely moved but I leapt decades with a few keystrokes. Yearbooks. Marriage certificates. Mugshots. I scoured thousands of pages of 1970s-era police files. I pored over autopsy reports. That I should do this surrounded by a half-dozen stuffed animals and a set of miniature pink bongos didn’t strike me as unusual. I’d found my searching place, as private as a rat’s maze. Every obsession needs a room of its own. Mine was strewn with colouring paper on which I’d scribbled down California penal codes in crayon.

It was around midnight on July 3, 2012 when I opened a document I’d compiled listing all the unique items he’d stolen over the years. I’d bolded a little over half the list: dead ends. The next item to search for was a pair of cufflinks taken in Stockton in September 1977. At that time the Golden State Killer, as I’d come to call him, hadn’t yet graduated to murder. He was a serial rapist, known as the East Area Rapist, who was attacking women and girls in their bedrooms, first in east Sacramento County, then snaking out to communities in the Central Valley and around San Francisco’s East Bay. He was young — anywhere from 18 to 30 — Caucasian and athletic, capable of eluding capture by vaulting tall fences. A single-story house second from the corner in a quiet, middle-class neighbourhood was his preferred target. He always wore a mask.

Precision and self-preservation were his identifying features. When he zeroed in on a victim, he often entered the home beforehand when no one was there, studying family pictures, learning the layout. He disabled porch lights and unlocked sliding glass doors. 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.

The Stockton case in September 1977 in which he’d stolen the cufflinks was his 23rd attack and came after a perfectly bracketed summer break. Drapery hooks scraping against a curtain rod awakened a 29-year-old woman in her bedroom in northwest Stockton. She rose from her pillow. Outside patio lights framed a silhouette in the doorway. The image vaporised as a flashlight found her face and blinded her; a force of energy rushed towards the bed. His last attack had been Memorial Day weekend. It was 1.30am on the Tuesday after Labour Day. Summer was over. He was back.

He was after couples now. The female victim had tried to explain the foul odour of her attacker to the reporting officer. She struggled to identify the smell. Bad hygiene wouldn’t account for it, she said. It didn’t come from his underarms, or his breath. The best the victim could say, the officer noted in his report, was that it seemed like a nervous scent that emanated not from a particular area on his body, but from his every pore. The officer asked if she could be more specific. She couldn’t. The thing was, it wasn’t like anything she’d ever smelled before.

As in other cases in Stockton he ranted about needing money but ignored cash when it was right in front of him. What he wanted was items of personal value from those he violated: engraved wedding bands, driver’s licenses, souvenir coins. The cufflinks, a family heirloom, were an unusual 1950s style and monogrammed with the initials N.R. The reporting officer had made a rough drawing of them in the margin of the police report. I was curious about how unique they were. From an internet search I learned that boys’ names beginning in N were relatively rare, appearing only once in the top one hundred names of the 1930s and ’40s, when the original owner of the cufflinks was likely born. I Googled a description of the cufflinks and hit the return key on my laptop.

It takes hubris to think you can crack a complex serial murder case that a task force representing five California jurisdictions, with input from the FBI, hasn’t been able to solve, especially when your detective work is, like mine, DIY.

My interest in crime has personal roots. The unsolved murder of a neighbour when I was 14 sparked a fascination with cold cases. The advent of the internet transformed my interest into an active pursuit. Once public records came online and sophisticated search engines were invented, I recognised how a head full of crime details could intersect with an empty search bar, and in 2006 I launched a website called True Crime Diary. 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. I share my theories with the loyal regulars who read my blog.

I’ve written about hundreds of unsolved crimes, from chloroform murderers to killer priests. The Golden State Killer, though, has consumed me the most. In addition to 50 sexual assaults in Northern California, he was responsible for 10 sadistic murders in Southern California. Here was a case that spanned a decade and ultimately changed DNA law in the state. Neither the Zodiac Killer, who terrorised San Francisco in the late 1960s and early ’70s, nor the Night Stalker, who had Southern Californians locking their windows in the ’80s, were as active. Yet the Golden State Killer has little recognition. He didn’t have a catchy name until I coined one. He attacked in different jurisdictions across California that didn’t always share information or communicate well with each other. By the time DNA testing revealed that crimes previously thought to be unrelated were the work of one man, more than a decade had passed since his last known murder, and his capture wasn’t a priority. He flew under the radar, at large and unidentified.

But still terrorising his victims. In 2001 a woman in Sacramento answered her phone in the same house where she’d been attacked 24 years earlier. “Remember when we played?” a man whispered. She recognised the voice immediately. His words echo something he said in Stockton, when the couple’s six-year-old daughter got up to use the bathroom and encountered him in the hallway. He was about 6m away, a man in a brown ski mask and black knit mittens who was wearing no pants. He had a belt on with some kind of sword in it. “I’m playing tricks with your mum and dad,” he said. “Come watch me.”

The hook for me was that the case seemed solvable. His debris field was both too big and too small — he’d left behind so many victims and abundant clues, but in relatively contained communities, making data mining potential suspects easier. The case dragged me under quickly. Curiosity turned to clawing hunger. I was on the hunt, absorbed by a click-fever that connected my propulsive tapping with a dopamine rush.

I wasn’t alone. I found a group of hard-core seekers who congregated on an online message board and exchanged clues and theories on the case. I set aside any judgments I might have had and followed their chatter, all 20,000 posts and counting. I filtered out creeps with iffy motives and concentrated on the true pursuers. Occasionally a clue, like the image of a decal from a suspicious vehicle seen near an attack, would appear on the message board, a bit of crowdsourcing by overworked detectives who were still trying to solve the case.

I didn’t consider him a ghost. My faith was in human error. He made a mistake somewhere along the line, I reasoned.

On the summer night I searched for the cufflinks, I’d been obsessed with the case for nearly a year. I favour yellow legal pads, especially the first 10 or so pages when everything looks smooth and hopeful. My daughter’s playroom was littered with partially used pads, a wasteful habit and one that reflected my state of mind. Each pad was a thread that started and stalled. For advice I turned to the retired detectives who’d worked on the case, many of whom I’d come to consider friends. The hubris had been drained from them, but that didn’t stop them from encouraging mine. The hunt to find the Golden State Killer, spanning nearly four decades, felt less like a relay race than a group of fanatics tethered together climbing an impossible mountain. The old guys had to stop, but they insisted I go on. I lamented to one of them that I felt I was grasping at straws.

“My advice? Grasp a straw,” he said. “Work it to dust.”

The stolen items were my latest straw. I wasn’t in an optimistic mood. My family and I were headed to Santa Monica for Fourth of July weekend. I hadn’t packed. The weather forecast was lousy. Then I saw it, a single image out of hundreds loading on my laptop screen, the same style of cufflinks as sketched out in the police file, with the same initials. I checked and rechecked the cop’s crude drawing against the image on my computer. They were going for $8 at a vintage store in a small town in Oregon. I bought them immediately, paying $40 for overnight delivery. I walked down the hallway to my bedroom. My husband was on his side, sleeping. I sat on the edge of the bed and stared at him until he opened his eyes.

“I think I found him,” I said. My husband didn’t have to ask who “him” was.

Extracted from I’ll Be Gone In The Dark by Michelle McNamara published by Faber & Faber, $29.99, out now.