But now, after nearly 15 years behind bars, Tankleff believes freedom may be within his reach. His investigator, Jay Salpeter, is going to court and naming the people he says killed Marty's parents.
"I'm very confident that I have the right people," says Salpeter. "I'm very confident."
Correspondent Erin Moriarty reports on the latest update to this 48 Hours Mystery.
"It was a wonderful childhood. I had more than the average kid," says Marty, who was adopted as a baby by Seymour and Arlene Tankleff.
Seymour, a savvy and tough entrepreneur, was grooming his son to follow in his footsteps. "I wanted to be a businessman," says Marty. "So I enjoyed being involved in all of that."
Marty says he knew everything about his father's businesses, including the trouble his dad was having with a partner in a bagel shop, Jerry Steuerman, who owed him a lot of money. "They essentially became enemy business partners," says Marty.
But despite that tension, both men continued to play in a weekly poker game. On Sept 6, 1988, it was Marty's father's turn be host. The game lasted into the wee hours and Steuerman was the last to leave.
The next morning, Marty says he woke to find his father near death. Marty called 911 and then he says he searched for his mother. He found her dead on her bedroom floor.
Det. James McCready, now retired, arrived an hour later. Seymour, who was bludgeoned and stabbed, was still alive and was rushed to the hospital. Arlene's body was in her room. "She was nearly decapitated, and it appeared to me that she had struggled with whomever had assaulted her," says McCready, who saw no sign of forced entry.
McCready, a homicide cop for 10 years, says he was immediately bothered by Marty's appearance: "He was sitting as calm as calm could be, with his hands clasped. I think he would have been crying. I think he would have been shaken, been very upset."
"I felt that they [police] were trying to help me and I was trying to help them," says Marty.
But McCready says he thought Marty was lying: "I get a feeling, it's not so much what is said. It's the way in which it's said."
Marty volunteered his suspicions that Jerry Steuerman, his father's partner, was somehow involved. He also agreed to talk more about that at police headquarters. But McCready thought he already had his man – Marty Tankleff.
Why? "One of the simplest old things in the world. Greed," says McCready.
For hours, 17-year-old Marty sat alone with McCready and his partner in a small, windowless room. And for hours, the detectives questioned him.
"It was the constant barrage, that 'Marty, we know you did it. Everything will be OK, just tell us you did it. We know you did it,'" says Marty. "And the on and on questioning, over and over."
Then, McCready did something that would change everything. He left the room, pretended to talk on the phone, and came back with news about Seymour Tankleff.
"Your father, they pumped him full of adrenaline and he came out of his coma, and he said that you did it,'" McCready told Marty. "I lied to him," he says, adding that it was OK to do that because "the United States Supreme Court says it is."
Marty begged to take a polygraph, but the detectives refused. Meanwhile, McCready's scheme worked. Marty began to wonder if he had blacked out, and if he had attacked his parents. Finally, he told the police what they wanted to hear: his confession.
How could they make him do it? "They have you believing," says Marty. "I was scared. I was disoriented. I was in shock."
McCready began to prepare a written statement in his handwriting. Although Marty never signed the statement, and almost immediately recanted his story, the detective had enough. Marty was arrested and charged with murder.
Jay Salpeter, a private detective, says Marty was forced to admit to a crime he didn't commit: "Det. McCready is a bad detective. It's just mind-boggling what this man did to this kid."
"He is guilty of waking up in the morning alive," says Marty's cousin and guardian, Ron Falbee. "And the minute Marty talks to anyone else outside of the cops, the first thing out of his mouth is, 'They made me say it.'"
Ron's not the only family member in Marty's corner. Marty's relatives believe his story, too, and they say that Det. McCready never even tried to talk to them.
"I never asked directly to speak to them. I didn't have to," says McCready. "What were they going to add to my case?"
"I was brought up to be a very non-emotional person," says Marty. "I was brought up to very much internalize emotions."
Police say Marty killed his parents to get money, but his family disagrees. They say that he wasn't supposed to get any money until he was 25, and he had just turned 17.
McCready says he wasn't aware of this, but he stands by his case: "Under the circumstances in this case, everything we needed to know we pretty much knew in the first day."
With his suspect behind bars, McCready thought he had the whole case all wrapped up. But a week later, as Marty's father Seymour lingered in a coma, the case took an unexpected turn. Seymour's business partner, the same man Marty had told the police to investigate, disappeared.
A police report, however, shows that McCready still refused to consider Jerry Steuerman a suspect. "He had nothing to do with that murder," says McCready.
Two weeks later, detectives found Steuerman in Long Beach, Calif., living under an alias. Steuerman returned home claiming his personal and financial problems caused him to flee.
One month after the Tankleffs were attacked, Marty was granted bail. "I totally believed he was innocent. I really believed he was innocent," says Marty's sister, Shari Mistretta, Seymour Tankleff's daughter from a previous marriage.
Their father, Seymour, who had been in a coma, died, never regaining consciousness. Marty was then charged with two murders, and nearly two years later, he went on trial.
"I think every emotion ran through me, scared, fearful, but I was also hopeful," says Marty. "I knew I was innocent, and I always believed that innocent men don't get found guilty."
By far, the most damaging evidence against Marty was his confession. But there was little physical evidence to back it up. None of Marty's hair or blood was found on his parents, and his mother, Arlene, had clearly fought her attacker. Yet, Marty had no cuts or bruises, only some swelling in his eyes from a nose job he got for his birthday.
The jurors also heard from Steuerman, the man the detectives had dismissed as a suspect. He admitted that he had owed Seymour Tankleff hundreds of thousands of dollars. What's more, he was angry that Seymour was demanding a share of the bagel store that Jerry was setting up for his son.
Under intense questioning, Steuerman snapped. But by contrast, Marty Tankleff was composed on the stand -- perhaps too composed, as he tried to explain why he would confess to something he didn't do.
After a week's deliberation, the jury reached its verdict: guilty. Marty was sentenced to 50 years to life. But 12 years into that sentence, Salpeter has stepped in and found witnesses who say they are ready to reveal secrets.