HAUPPAUGE — A man imprisoned more than 17 years in the 1988 killing of his parents will not face a second murder trial in the case, a prosecutor said today.

“It is no longer possible to reasonably assert that the case … would be successful,” the Suffolk County district attorney, Thomas Spota, said.

He said his office will formally drop the indictment against Martin Tankleff in the 1988 deaths of his parents, Seymour and Arlene Tankleff, at a scheduled January 18 court conference in Riverhead.

Mr. Spota also announced he will ask Governor Spitzer to appoint a special prosecutor to investigate claims made by Tankleff and his defense attorneys that Seymour Tankleff’s business associate or others may have been involved in the killings.

A spokeswoman for Mr. Spitzer had no immediate comment on Mr. Spota’s request for a special prosecutor.

Tankleff, now 36, was convicted in 1990 of killing his parents in their home in the exclusive Belle Terre neighborhood overlooking Long Island Sound. He was released after serving 17 years of a 50-year term after a New York appeals court found that new evidence uncovered by private investigators suggested the Tankleffs may have been killed in a business dispute.

His case attracted national attention after high-profile lawyers took the appeal amid a public relations campaign by members of the Tankleff family who insisted that someone other than the teenager had killed their relatives.

The panel said it was “probable” that a new jury would render a different verdict if given a chance to reconsider all evidence now available. It did not find, however, that Tankleff was innocent and instead directed that a new trial be held.

Tankleff was released last week on $1 million bail and immediately started working on what he expected would be his upcoming trial.

Tankleff was convicted of intentional murder in the death of his father, but was found guilty of “depraved indifference to human life” in his mother’s death.

Mr. Spota said today that he opted to drop the charges because the law defining “depraved indifference to human life” has changed in the ensuing years and could no longer apply to the killing of Arlene Tankleff.

And he said double jeopardy laws prevent him from retrying Tankleff on the intentional murder count for his mother because the 1990 jury acquitted him of that charge, Mr. Spota said.

Without a case against Tankleff in the death of his mother, “the defense will surely argue that all evidence with respect to Arlene Tankleff’s murder be excluded from the trial of Tankleff for the murder of his father,” he said.

He said while prosecutors could argue that the evidence be admitted, “I believe that attempting to try half of the case against Tankleff is futile and I will not do it.”

Tankleff was starting his first day as a high school senior in September 1988 when he says he discovered the bodies of his mortally wounded parents. Arlene Tankleff was declared dead at the scene and Seymour was taken to a hospital with critical injuries.

Detectives questioning the teenager falsely told him that his father had awoken from a coma and implicated him in the crime.

“Could I have blacked out … and done this?” Tankleff said, according to court records. “Could I be possessed?”

“Marty, I think that’s what happened to you,” a detective said.

Tankleff then confessed to the killings, according to police, but almost immediately recanted.

For years, Tankleff and his supporters have claimed that the murders were the work of a Long Island businessman, Jerry Steuerman, who they said owed hundreds of thousands of dollars to the family and was at the home for a poker game on the evening of the killings.

Within days of the murders, Mr. Steuerman faked his own death, changed his appearance and fled to California. He was never named as a suspect and has repeatedly denied any involvement in the killings. Mr. Steuerman testified as a prosecution witness at Tankleff’s trial.

Investigators hired by the defense team tracked down a number of witnesses who claimed to be able to implicate a career criminal, Joseph Creedon, in the killings.

During appeal hearings in the case, Creedon acknowledged that he had a violent past that including working as a debt collector for Mr. Steuerman’s drug-dealing son, but said he never killed anyone.

Mr. Spota said although the defense theory of a hit being taken out on the Tankleffs is “totally inconsistent” with the evidence, he will ask the governor to appoint a special prosecutor to investigate the claims.