In the movies and on television, the monomaniacal detective in pursuit of a cold case killer is not only a cliche but a visual cliche. There’s always a corkboard disappearing beneath a sinister profusion of photographs and maps — pushpins and crisscrossing twine connecting persons of interest to victims and victims to other victims and evidence to crime scenes from which the yellow tape has long since fluttered away. Think Robert Graysmith (Jake Gyllenhaal) in Zodiac or Rust Cohle (Matthew McConaughey) in the first season of True Detective. Just who is the real madman here?

Michelle McNamara’s life had become just such a corkboard when she died on April 21, 2016, of an accidental overdose. The author of the enormously popular True Crime Diary blog, McNamara was representative of a burgeoning subculture of forensic Batmen — obsessive amateurs, but amateurs competent enough to form reciprocal ties with active and retired law enforcement. Her No. 1 quarry was a prolific serial rapist and murderer active near Sacramento, California, from the mid-1970s to the ’80s. She’d named him herself: the Golden State Killer.