(ABC NEWS) -- Gary L. Stewart was interested about his lineage.
The Baton Rouge, Louisiana, man was adopted as a boy. When he was 39 years old, his birth mother contacted him.
Stewart wanted to find out more about his biological parents, so he started digging. He spent 12 years studying records and clues. That search led him to an interesting conclusion -- that his father, who died in 1984, was the Zodiac Killer.
"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."
His father’s fingerprints and handwriting were eerily similar to that of the infamous criminal, a serial killer credited with killing five people in the late 1960s in California, Stewart said. The killer earned his nickname through the taunting letters he sent to police and media.
Through the years, numerous people have claimed that their relatives were responsible for the murders, and Stewart is next -- publishing his research in the memoir, “The Most Dangerous Animal of All,” which was released Tuesday by Harper Collins. The book was co-written by Susan Mustafa.
Within 24 hours of its release, the book became one of the most popular titles on Amazon.com, drawing intrigue from true-crime enthusiasts.
Bryan Hartnell, who survived an attack by the Zodiac Killer, expressed reservations about Stewart’s claims to CNN.
“I somewhat follow the news, but there has been no time in the last 40+ years when someone was not [stirring] the pot," Hartnell wrote in an email.
At this point, the Zodiac Killer case remains open.