Shortly before his murder last year, Nat Masselli told family and friends that he was working with "the feds" in hopes of getting his father out of prison, according to accounts obtained by Bronx authorities.

The son of a reputed mobster caught up in the federal investigation of Labor Secretary Raymond J. Donovan, Masselli, 31, was gunned down in gangland style in the north Bronx on Aug. 25, 1982. Mistakenly or not, his relatives blame the FBI.

"They led him into a dangerous situation," declared one family member who asked not to be identified. "The FBI led him down the garden path."

A special prosecutor spent nine months last year investigating alleged ties between Donovan, a former New Jersey construction company executive, and organized crime. Donovan repeatedly denied the allegations under oath.

Federal officials flatly denied at the time of Masselli's death that he had been an informer for them.

But the stories from family members and friends, which have Masselli telling how he was "wired" by FBI agents to gather evidence in the Donovan investigation, seem to contradict this and to shore up testimony this week at the trial of one of Masselli's alleged killers.

The testimony was that Masselli was killed when the two reputed Mafiosi in the car with him discovered that he was "wired" for the meeting, or at least was carrying a hidden tape recorder.

The stories from family and friends are said to include one from Nat Masselli's fiancee, Jo-Ann Sarno. She is reported to have told investigators for Bronx District Attorney Mario Merola that FBI agents visited Masselli at his apartment house in Scarsdale, N.Y., several times before he was killed, and that he once told her of a time when "they federal agents put a body wire on me" in an effort to tape the remarks of an unidentified individual at a restaurant-pub in Scarsdale.

Masselli, it was said, laughed as he told the story because the wire fell out of his clothes and "the feds were ----ed off about it," so much so that he would have to try again.

One of Masselli's sisters, sources said, recalled similarly that he told her one day that their father, William Masselli, had disclosed to an outsider what Nat Masselli "was doing with the FBI." Masselli then expressed his annoyance with his father. "I'm doing this for him," he was quoted as saying, "and he's ----ing the whole thing up."

Nat Masselli had helped FBI agents working for Leon Silverman, special prosecutor in the Donovan investigation, record several phone conversations with a lawyer for Donovan's company in May, 1982. But, officials declared after his death, Masselli "never wore any electronic surveillance or eavesdropping devices on his person."

The trial here this week of Salvatore Odierno, 68, one of Masselli's two alleged murderers, has already cast doubt on those disclaimers. The state's star witness in state Supreme Court, James Toohill, has said that Odierno told him in jail last fall that Masselli was shot because he had a tape recorder on his person.

Toohill, who was testifying under the assumed name of William Burns, identified himself from the witness stand Tuesday. A cellblock companion of Odierno's at Riker's Island Prison Hospital last fall, Toohill said Odierno told him that he and his companion, Philip Buono, 68, had been trying to persuade Nat Masselli not to testify in the Donovan investigation.

According to Toohill, Odierno, a reputed loan shark, offered to "wash out" a number of loans to the Massellis, but plotted with Buono to kill Nat Masselli if he balked.

Odierno's lawyer has vigorously assailed Toohill's credibility. The defense lawyer, Louis Aidala, sought to portray the witness as a longtime government informer who habitually inflated his importance and frequently lied about his accomplishments and activities.

Under three days of cross-examination, Toohill estimated he had earned about $15,000 as an informer for state and federal agencies since 1967. He was paroled last July from a one-to-three year sentence for trying to sell information about a Manhattan grand jury's 1977 investigation of the Sotheby Parke Bernet auction house to Sotheby's president for $100,000.