OSTON, Nov. 4 (AP) - A man who died in prison after serving 30 years for a mob-related murder that the authorities now acknowledge he did not commit has been posthumously exonerated by prosecutors who say he was framed.

The man, Louis Greco, who died in prison in 1995, was set up by Joseph Barboza, a hit man who became a government witness, the Suffolk County district attorney's office said in a motion filed quietly in Suffolk Superior Court in September. Also in September, a judge ruled that a civil suit in the case could go forward. The dropping of charges was reported on Thursday by The Boston Herald.

The case was part of a series of embarrassing episodes involving the Boston office of the Federal Bureau of Investigation, whose relationship with mob informers has been the subject of a Congressional inquiry.

"It appears that justice may not have been done," Mark Lee, an assistant district attorney, said in the motion exonerating Mr. Greco. The motion also cites "legal and ethical considerations."

Mr. Greco always maintained he was in Florida on March 12, 1965, when Edward Deegan was gunned down in an alley. Mr. Greco was 78 when he died in a prison hospital of colon cancer and heart disease.

In 2000, a Justice Department task force uncovered secret F.B.I. memorandums showing that Mr. Greco and three co-defendants, Peter J. Limone, Joseph Salvati and Edward Tameleo, had been wrongly convicted based on perjured testimony.

The next year, a judge exonerated the surviving co-defendants, Mr. Limone, who spent 33 years in prison, and Mr. Salvati, behind bars for 30 years. Mr. Tameleo died in prison in 1985. The judge found that F.B.I. agents hid testimony that would have cleared the men because they wanted to protect Mr. Barboza, who became a witness in three mob trials.

Mr. Limone, Mr. Salvati and Mr. Greco's family sued the government for malicious prosecution, wrongful imprisonment and other claims.

In her Sept. 17 ruling allowing the suits to go forward, Judge Nancy Gertner of Federal District Court, rejected the government's argument that no laws allowed the men to sue at the time they went to prison. Congress did not vote to waive immunity to such claims until 1974.

Howard Friedman, a lawyer who represents Mr. Greco's son, Edward, told The Herald that the district attorney's decision to drop the charges would aid his lawsuit. "This was an innocent man who was framed,'' Mr. Friedman said, "and the most amazing part is the government knew it."