Part II: Prime Suspect

Marty Tankleff says he thinks of his parents every day: "I think about all the good times that we had together. We always used to have family affairs. I used to love having those."

Marty's relatives have never wavered in their faith in his innocence. But there's one person, his half sister, Shari Mistretta, who's 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: "Short of somebody threatening my children, 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."

"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.' I don't know how. I just did."

"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 to him."

Marty says that the police asked him if he could have blacked out and committed the crime. "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 is convinced that Marty's confession is wrong because the details are so wrong.

"His confession is actually evidence of his innocence," adds Ofshe. "There are no aspects of it that are accurate, because 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.

But if Marty didn't kill his parents, then who did?

Using old-fashioned detective legwork, Salpeter says he 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?"

Salpeter is determined to get Marty out of prison by proving who really killed his parents. "There isn't a greater gift that I feel that I could do for a human being," says Salpeter. "It's a tough case, and this kid didn't do it."

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 was with two men, Joseph Creedon 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."

Today, both Kent and Creedon deny they were involved in crime, even though Harris passed a polygraph test arranged by Marty's defense team. "He was in shock over the allegations being made against him," says Creedon's attorney, Anthony LaPinta, who adds that his client will not take a polygraph test.

Marty still believes Jerry Steuerman was somehow connected to the crime. Creedon denies knowing Jerry Steuerman, but he did know Jerry's son, Todd, a convicted drug dealer.

Jerry Steuerman refused to talk to 48 Hours, but both he and his son deny they had anything to do with the Tankleff murders. Steuerman is still in the bagel business, but he's moved from New York and now lives in an affluent community in Boca Raton, Fla.

Salpeter decided to investigate the man dismissed as a suspect so many years ago. But will there be enough new evidence to get Marty another day in court? "I'm hopeful that now that all the new evidence has come out, that the truth will come out," says Marty. "[I've lost] family, freedom – things I could never get back."

But the Suffolk County New York D.A.'s office is fighting Marty. "The evidence presented does not exonerate Marty Tankleff," says Assistant District Attorney Leonard Lato, who wrote a report, released last December opposing Marty's request for a new trial.

"Perhaps one day Marty Tankleff will be able to present new evidence, credible evidence of his innocence. But he has not done so through Glenn Harris," adds Lato, who claims that Harris has changed his story several times.

How does he know? While Harris was still in jail, Lato asked other inmates to secretly tape him while pumping him for information. As Harris told and re-told his story, some details did change.

Even with his inconsistencies, Harris isn't the only person who puts Creedon at the Tankleffs home the night they were killed. Over the years, Creedon has told others he was there, and Lato doesn't dispute that.

"I'm conceding that Creedon admitted, notwithstanding his denials, that he had some involvement in the Tankleff murders," says Lato, who believes that Creedon took credit for the crime to enhance his violent reputation.

However, Kent believes Creedon is capable of murder

Leto believes that Marty alone is responsible for the crime. And that has Marty's family fuming. Even his estranged sister, Shari, who once believed her brother was guilty, now has serious doubts.

"I don't know what to say at this point. I'm back on that fence again, you know," says Shari. "I'm questioning a lot of the things that have happened and the things that are going on, and I would like some answers."

"I'm really hopeful that after she sees all the new evidence that who really killed her father and my mother that she'll see the truth and she'll believe me," says Marty.

This spring, a judge will decide whether Marty gets a new hearing. "I think about tomorrow," says Marty. "I think about just going one day at a time."

Even if Marty gets a new hearing, he still faces an uphill battle. While the district attorney certainly isn't buying Harris' story, that Creedon and Kent killed the Tankleffs, he does say that even if it was true, it still wouldn't necessarily clear Marty. In fact, he'd have to prove that he wasn't also part of the plot.

Marty's family, meanwhile, has vowed to keep fighting.