He claims he's discovered the identity of one of the most notorious serial killers in American history. The Zodiac Killer murdered at least six people in the San Francisco area between 1969 and 1972. He was never caught.
The brutal Zodiac murders were portrayed in the 2007 movie Zodiac with Robert Downey Jr. and Jake Gyllenhaal.
The killer even threatened to massacre children on school buses, forcing police to give kids armed escorts. He taunted investigators with letters, some containing bizarre coded messages, all signed with the symbol of a circle with two lines crossing in it.
Now, Gary Stewart says he's cracked the case and the Zodiac Killer was his own biological father.
"The Zodiac always insisted that if you crack the cypher, you will have my identity," said Stewart.
Watch What Stewart Told INSIDE EDITION Convinced Him His Father Was the Zodiac Killer
INSIDE EDITION's Les Trent asked Stewart, "Is there any doubt in your mind that your father was the Zodiac Killer?"
"None whatsoever," answered Stewart.
He has written about his amazing discovery in a new book, The Most Dangerous Animal of All: Searching for My Father and Finding the Zodiac Killer.
Stewart was found abandoned in a stairwell when he was a baby and raised by adoptive parents. He searched for his biological father and discovered he was a San Francisco antique dealer named Earl Van Best Jr., who died in 1984.
He was stunned when he saw this police sketch of the Zodiac Killer while watching a TV show on the case.
"I see the wanted poster for the Zodiac Killer from the San Francisco Police Department from 1969 and my heart stopped. The hair stood up on the back of my neck," recalled Stewart.
The likeness is stunning. Superimpose the sketch over the photo of his father and they're virtually an exact match.
Gary investigated further and says he even found his father's full name, Earl Van Best Jr. buried in several coded messages.
"E-A-R-L-V-A-N-B-E-S-T-J-U-N-I-O-R. He spelled it out completely. It was all about his name. It wasn't a secret message," said Stewart.
He also had handwriting experts compare the Zodiac letters with his father's handwriting. One expert concluded that it's virtually certain that Earl Van Best Jr. is the writer of the Zodiac letters.
Stewart says it's disturbing to think his own father may have committed such heinous crimes, saying, "There are times even when my wife looks at me and I can tell what she sees because I see that reflection in the mirror."