Marty's relatives have never wavered in their faith in his innocence. But there's one person, his half sister, Shari Mistretta, who has spoken out against Marty after his conviction.
"I feel like I'm the black sheep of the family," says Shari. "I saw a very manipulative young man who was able to manipulate two adults, Arlene and Seymour, into believing that he was the quintessential child."
Shari says she can't understand why Marty confessed to the crime: "No matter what you ever say to me, you couldn't make me confess to something that I didn't do."
But Marty claims that the police made him do it. "It's like having an 18- wheeler driving on your chest and you believe that the only way to get that weight off your chest is to tell the police whatever they want to hear … even admitting to a murder."
The confession is the most troubling part of Marty's case. The detectives chose not to record it, but if they had taped it, some experts believe, we'd be able to see how police techniques can lead an innocent person to say he's guilty.
"It's a false confession … Happens all the time," says Richard Ofshe, an expert in interrogation tactics who's working on Marty's appeals.
Ofshe says that Marty was a sheltered teenager and believed in the honesty of police officers. "Det. McCready said my father said it was me, and I believed that," says Marty. "It was at that point that I said, 'Maybe I could have done it.'"
"What happened in that room is modern police interrogation directed at a vulnerable child who just discovered his parents dead," says Ofshe, who believed that Marty was badgered for hours and began to question his own memory. "He knows he didn't do it, but he's confronted with a police officer who's lying to him, and skillfully lying."
Ofshe believes that after being badgered for hours, Marty began to question his own memory -- and the police gave him a way out. "All of a sudden there is a way of reconciling it. And that is you had a blackout, because of some psychological condition you got that impairs your memory," says Ofshe, who says he is convinced that Marty's confession is wrong because the details are so wrong.
"His confession is actually evidence of his innocence," adds Ofshe. "It's very specific about how the crime happened and all those very specific things are disproven."
For example, Marty told police that he used a barbell and kitchen knife as murder weapons, but not a trace of blood was found, even when they were microscopically examined. Also, the forensics team found bloody glove prints at the scene, but Marty never mentioned wearing gloves, and those gloves were never found.
Jay Salpeter, the former New York City cop who is working on Marty's case, says that once McCready had the statement from Marty, he could no longer investigate any other suspect: "How is this detective going to find another murderer when Marty Tankleff confessed to him. How is he going to get out of that?"
"I'm not taking a confession from an innocent man," says McCready. "I would never do that."
But a year after Marty's arrest, a state investigation found that in Suffolk County, where McCready worked, there was an "astonishingly high" 94 percent confession rate, which it said "provokes skepticism."
McCready makes no apology: "We do a good job and we are very good at doing it."
Salpeter, however, has tracked down a man, Glenn Harris, who told him he's kept a secret for 14 years and that he knows who really killed Marty's parents.
"He asked me for the truth and I told him the truth," says Harris. "He gave me the opportunity. Who else could I go to?"
Harris, a career criminal, was serving time for burglary when Salpeter tracked him down through a lead that came after Marty's conviction. Harris says that on a night in September 1988, he was the driver on the way to what he thought would be a home burglary. He said he was with two men, Joseph Creedon (known on the street as Joey Guns) and Peter Kent, who also had long criminal records.
In a notarized affidavit, Harris says he drove them to an upscale neighborhood and "parked his car... where Creedon told me to stop." When they returned to the car, Harris says he knew something happened, something worse than just a burglary: "Their demeanor, their behavior, it wasn't normal. …Extremely nervous, winded. Creedon's anxiousness to get out of there."
Harris says he later watched Kent "burning his clothes," and when he heard about the Tankleff murders, he "put two and two together," but kept quiet. "I had no right being up there," he says. "I was using drugs. I was just out on parole."
Creedon reportedly told other people he was involved in the crime, although he and Kent now both deny it. But Harris passed a polygraph test arranged by Salpeter, Marty's investigator.
Salpeter also says that Creedon is linked to Jerry Steuerman, the man that police had always dismissed as a possible suspect. Steuerman was the last person to leave the poker game before the murders. Creedon denies knowing Steuerman, but he did know Jerry's son, Todd, a convicted drug dealer.
"My scenario is that Seymour is sitting at his desk. Jerry Steuerman is talking to him, keeping Seymour's attention on Jerry," says Salpeter. "At this point, behind Seymour, coming through the door, Joe Creedon, Peter Kent, and they took Seymour out and then went for Mrs. Tankleff."
Steuerman refused to talk to 48 Hours, but both he and his son deny they had anything to do with the Tankleff murders. Steuerman now lives in an affluent community in Boca Raton, Fla.
The discovery of new evidence is a major break for Marty. He has been granted a hearing. If the judge at that hearing finds that the new evidence would have caused the jury to vote in a different way, Marty will get a new trial, and have a real shot at winning his freedom.
"This is the first time that the truth is gonna be coming out in the courtroom, and we're gonna be bringing in the people to give us the truth," says Salpeter.
As the hearing begins, Marty's family and his lawyers, who are working pro-bono, are thrilled to be back in court.
They have new evidence, but it's evidence that will not go unchallenged. Assistant District Attorney Leonard Lato is fighting to uphold Marty's verdict. "They must come forward with clear evidence to show that that verdict is incorrect," says Lato.
Lato claims Creedon took credit for the murders only to enhance his violent reputation. But a woman who met Creedon at a party, Karlene Kovacs, says Creedon sounded all too sincere when he told her about the crime and that she believed he was telling the truth.
Kovacs, one of the first witnesses to testify, gave a sworn affidavit nine years ago. She believes her information was "thrown in the garbage."
Glenn Harris, however, is one witness that everyone is waiting to hear. He is brought to court from jail, where he is serving time for a parole violation. But when Harris takes the stand, he surprises Marty's supporters by refusing to testify on the grounds that he might incriminate himself.
Lato says he wasn't surprised, because there were inconsistencies in Harris' story: "In my view, he isn't testifying because he doesn't want to get up on the stand, subject himself to cross examination and be exposed as a liar."
Salpeter believes that Harris is afraid that the district attorney, who refused to give him immunity, will charge him with the Tankleff murders: "I think the district attorney is doing everything possible to muzzle Glenn Harris."
It's a terrible blow to Marty and his family -- but the case isn't over yet.
Marty's fight to get a new trial will have to be waged without his star witness, Glenn Harris. But even without his testimony, Harris' sworn affidavit has helped the defense team.
Meanwhile, Salpeter has tracked down other witnesses who appear to corroborate the theory he presented in court: Creedon, Kent and Steuerman were the men behind the murders.
"New witnesses have come forward, and we're hopeful that we're going to bring Marty home," says Salpeter.
The witnesses include the Rev. Ron Lemmert, a Catholic priest who testifies, with Harris' permission, that Harris told him the same story he told 48 Hours. "He really wanted to do the right thing, but he was a man who was terrified," says Lemmert.
But perhaps the most dramatic new witnesses are those who tell the court that Creedon tried to involve them in his plot to kill Seymour Tankleff.
Joe Graydon, one witness, claims he and Creedon made an earlier, failed attempt to ambush a man Graydon now believes was Tankleff.
Another witness, Rill Ram, a convicted drug dealer, not only corroborates Harris' story that he was with Ram the night of the murders but says he spoke to Creedon that same night.
"He said, 'There's some money in it for me if we go there,' and just, you know, he's going to threaten the guy or rough him up," recalls Ram, who turned Creedon down.
But he says Harris went along with Creedon. Ram told 48 Hours that "when I saw him the next day, he was completely distraught...just shooken up ... just scared to death."
"I really didn't wanna be involved," says Ram.
Marty says he was surprised by the honesty of these witnesses: "That after all these years, that they would come forward and admit their involvement in such brutal crimes."
Lato, however, sees these witnesses in a different light: "The point is, when people have criminal records, are still committing crimes, those things affect their credibility."
But Salpeter says it makes sense that the people willing to work with Creedon would have records themselves: "These were the people that were there. These are the people that knew."
Other witnesses had no record at all, but Lato claims he has found holes in the testimony of every significant defense witness.
When it's the state's turn to present witnesses, the hearing becomes almost surreal. Kent, Creedon's alleged accomplice in the Tankleff murders, comes to court on behalf of the district attorney's office.
Kent denies he had anything to do with the murders, but he admits that he thinks Creedon is capable of murder,just not the Tankleff murders. "I know that, because he was not with me that night, and we didn't do these with Glenn. It never happened," he says.
Creedon, who has been convicted of rape and grand larceny, denies ever killing anyone. But on the stand, he admits to a life of violence, collecting money for drug dealers.
Lato believes that "in terms of the evidence, that there's no evidence connecting them [Creedon and Kent] to the crime at all, no credible evidence."
But Salpeter says the district attorney is doing his best to "suppress the truth from coming out." He also says that Creedon and Kent are getting the same free ride today that Steuerman got when the Tankleffs were murdered.
With legal wrangling and court delays, Marty's hearing has dragged on for seven long months. A parade of witnesses have now come forward to support Marty's bid for a new trial.
But Tankleff's future now rests with the judge.
"We have presented a significant enough, compelling evidence for me to be granted a new trial," says Marty. "That's what I know. What the judge will do, I guess we'll have to wait and see."
The judge is expected to rule on his case this spring.