After being contacted by his birth mother at age 39, Stewart began investigating his biological father, . Stewart claims that Van Best Jr. is the Zodiac Killer based on several pieces of circumstantial evidence:
A handwriting expert initially suggested that Van Best Jr.'s writing on a marriage certificate matched the Zodiac's cryptic letters. The Most Dangerous Animal of All
Stewart claimed to find his father's name, "Earl Van Best Jr.," hidden within the Zodiac’s famous unsolved cryptograms. After being contacted by his birth mother at
Van Best Jr. was a convicted pedophile with a history of predatory behavior, providing a psychological profile that Stewart argued fit the killer. The FX Docuseries and Debunking Stewart began investigating his biological father
Stewart noted a striking similarity between his father's mugshot and the 1969 police sketch of the Zodiac.