Mathews went back and painstakingly pored over every one of the 10,000 pages in the case file, revisiting the spot where Adam was killed and the pond where his head was found. He concluded that the lead detective on the Walsh case, a man named Jack Hoffman, became convinced, despite compelling evidence, that Ottis Toole didn't do it.
"Ego was involved and tunnel vision," Mathews said of Hoffman. "And the tunnel vision is that you come up with your own hypothesis as to what took place and you work that case to validate your own hypothesis."
Hoffman declined a request for comment by "Nightline."
After combing through the case file, Mathews made a major discovery: a roll of film from the crime scene of Ottis Toole's car -- it was film that the original detectives never bothered to have developed.
Pictures show bloody footprints on the driver's side of Toole's car. On the rear floorboard of the car -- where Toole admitted to tossing Adam's severed head -- pictures show the bloody outline of a face.
"I have a blood transfer from Adam's face onto the carpet -- you can actually see his image. It's as clear as the shroud of Turin, Veronica's veil. It's clear," Mathews said.
He showed the picture to the Walshes.
"To me, it was the one thing that a mother knows, is that this is their child, that this picture is their child," Reve Walsh said. "This is the piece of evidence that ties everything together for me and I can go to my grave knowing that not only that I did everything I could but that I found my answers in that photo."
In 2008, based on the evidence presented by Mathews -- who is one of the co-authors of "Bringing Adam Home," -- the new Hollywood Police Department Chief Chad Wagner officially closed the case, definitively naming Ottis Toole, who had since died in prison, as the man who killed Adam. Wagner apologized to the Walshes for mistakes that he said were made early in the investigation.
Today, the Walshes still live in Florida, where they have a horse farm and three children, all born after Adam.
"Nothing is going to bring Adam back. Nothing is going to bring him back so the most we could hope for would be to have peace, knowing that we know what happened, we know the end result," Reve said. "And the puzzle is finished."