For the last 50 decades, the identity of the Zodiac Killer has remained one of the most puzzling cold cases in American true-crime culture. With countless suspects emerging and fading into obscurity, the serial killer struck terror throughout Northern California’s Bay Area in the late 1960s, inspiring countless podcasts, books, and even the 2007 Zodiac film starring Jake Gyllenhaal, Mark Ruffalo, and Robert Downey Jr.

For those who are unaware of the cold killer, the Zodiac Killer is an unidentified serial murderer who terrorized Northern California. He is infamous for his cryptic letters, taunting authorities with symbols, ciphers, and claims of several unsolved murders. The Zodiac Killer also claimed to have killed 37 people, but only five murders and two attempted murders have been linked to him. Despite extensive investigations, the Zodiac Killer’s identity remains a mystery, except, for Arthur Leigh Allen, a former elementary school teacher and convicted sex offender,...