March 2, 2011 -- Reve Walsh still remembers the last conversation she had with her six-year-old son Adam at a Florida shopping mall.
"I said, 'I'm going right over here to the lamp department,' and he said, 'Okay, Mommy, I know where that is,'" Walsh told "Nightline."
Reve Walsh rarely speaks publicly of her son's murder, but chances are you've heard of Adam Walsh, or at least his father: Reve's husband, John Walsh became the public face of the missing children's movement after Adam's death and is the longtime host of "America's Most Wanted."
Now, as a new book, "Bringing Adam Home," tells the story of the Adam Walsh case, the Walshes sat down to speak with "Nightline."
Adam Walsh was abducted from a suburban Florida shopping mall on July 27, 1981. Two weeks later, his severed head was found floating in a canal. The rest of his body was never recovered.
"When all this was going on, I just thought to myself, I can't wait till it's 20 years or 25 years," Reve said. "I don't care about getting old, I just want this experience of my life to be that far away from me because I thought it would be different. It's really not."
From the beginning, the Walshes were disappointed with the police department in Hollywood, Fla.
First, the Walshes say the cops didn't take the missing persons case seriously enough. Then they say the police botched the murder investigation.
"It was one nighmare after another," John Walsh said.
The prime suspect was Ottis Toole, an admitted serial killer who was in prison for many other crimes.
"You keep killing and killing and it gets easier and easier and it don't really bother you," Toole once told Texas law enforcement.
Toole made 24 documented confessions to killing Adam Walsh, including a 1984 videotaped interview with Texas Rangers. During the interview, he said, "I kind of feel bad about that Adam Walsh kid."
"That is," he said, "the youngest child I ever killed."
Toole even wrote the Walshes a letter in 1988, demanding $5,000 in exchange for telling them where the rest of Adam's remains were.
Bloody Faceprint: 'A Mother Knows'
"I read that letter and went into the bathroom and threw up," John Walsh said. "I said, 'This son of a bitch. We have to figure out a way for him to tell us what happened."
In the decades that followed, John became a celebrity and "America's Most Wanted" helped capture more than 1,000 criminals. But Adam's case remained open -- Toole recanted his confessions about Adam's murder three times and charges were never filed against him, with authorities citing at lack of evidence.
As the years went by, Reve Walsh grew increasingly frustrated.
"I cannot go to my grave not doing everything I can possibly do," she said. "We are going to get a cold case detective, we are going to start from the beginning, and have this man work this case. I don't care if you go to work just to pay the bill of this private investigator.' And John said, 'I know the guy.'"
That guy was Joe Mathews, a retired police detective who had briefly been involved in the Walsh case early on and was critical of how it was handled.
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."