Instead, Stewart, the 51-year-old owner of a Baton Rouge-based industrial cleaning company, believes he uncovered new evidence that proves his dad was the notorious Zodiac serial killer, linked to five deaths in Northern California in the late 1960s.
"This is the last thing I wanted to find out, believe me," Stewart tells PEOPLE in an exclusive interview.
The details of Stewart’s 12-year search, involving interviews with handwriting specialists, forensic scientists and over 500 people, is detailed in his book The Most Dangerous Animal Of All, released Tuesday by HarperCollins. The book, vetted by the publishing house’s attorneys, has been kept a tightly guarded secret for the past year.
"I'm really hoping this will bring some closure to the families of my father's victims," says Stewart, who claims to have spent a decade unsuccessfully trying to get the San Francisco Police Department to compare his DNA with evidence they have on file for the Zodiac.
Bettmann / Corbis
Adopted as an infant by a loving family, Stewart never knew the identity of his birth parents until his birth mother – Judith Gilford – reached out to him in 2002. He soon learned that she was 14 when she ran away from home with a 27-year-old rare book dealer named Earl Van Best Jr., later giving birth to his child in New Orleans in February 1963 when the two were on the run from the authorities searching for Gilford, a minor. Against the wishes of Gary's frightened, confused teenage mom, Best abandoned their month-old son in a Baton Rouge apartment building.
Best was arrested on charges of raping a minor, along with document and wire fraud, and spent several years in a maximum-security facility for the criminally insane and San Quentin.
He was paroled in July 1965 and the first Zodiac murder occurred in December 1968.
Gilford cut off all ties with Best after his arrest. Stewart claims that his father's victims all resembled his birth mother and believes that the motive behind the killings was "revenge."
Best died in 1984. Investigators have never solved the Zodiac murders and it's currently considered a cold case by the San Francisco Police Department.
"I still don't have all the answers,” says Stewart, whose search for his father's identity consumed him for the past decade. "I never will. But I've got all the answers I want and I’m truly ready to get on with my life."