A new book released by HarperCollins claims that author, Gary L. Stewart, of Louisiana, discovered his father was the Zodiac Killer while he was on a 10 year search for his biological father.

To this day no one really knows the true identity of the Zodiac Killer. In his boastful notes to police, the killer said he had actually killed 37 people. In his letters, the Zodiac Killer included bloody piece of clothes that were taken from the victims.

Over the course of the ten year search, Stewart kept a journal, carefully documenting all of his discoveries. The journal is what became the book The Most Dangerous Animal of All: Searching for My Father … and Finding the Zodiac Killer. Best, who is now deceased, had a police record. In San Francisco, Stewart’s father wrote bad checks and engaged in criminal forgeries. One thing that stood out to Stewart when looking at a mug shoot of his father, was the striking resemblance to a sketch of the Zodiac Killer. To him, they looked alike.

“I never set out to prove my father was a serial killer,” Stewart told ABC News affiliate WBRZ, “and when I discovered this horrible truth, I set out to prove that I was wrong about my assumption.”