The FBI recruited a mafia enforcer to help solve the slaying of three civil rights workers by the Ku Klux Klan in Mississippi in the 1960s, according to evidence provided in Brooklyn state court by a mobster's moll. In the 1988 film Mississippi Burning that recreated the events, the case was broken when the FBI brought in a black agent to put the frighteners on a Ku Klux Klan member.

But Linda Schiro told the court it was her boyfriend, Gregory Scarpa, who had kidnapped a klansman, put a gun in his mouth and forced him to reveal the spot where the three had been buried. The FBI until that point had been engaged in a fruitless search for the civil rights workers, facing silence from the klansmen and the white community in general.

James Chaney, Michael Schwerner and Andrew Goodman were beaten and shot by a gang of white supremacist klansmen and buried in an earthen dam near Philadelphia. Rumours about the involvement of Scarpa have circulated for years and a report appeared in the New York Daily News a decade ago based on federal sources but this is the first time the claim has surfaced in open court.

Scarpa, who Ms Schiro said had boasted of involvement in 20 killings and was nicknamed the Grim Reaper, had been a capo in the Colombo crime family. He died in jail in 1994. The Mississippi case attracted a lot of political and media attention. Scarpa, according to Schiro, "said that [the FBI director J Edgar] Hoover at the time was getting a lot of pressure about the bodies, where they were."

Schiro, who met Scarpa in a bar in 1962 when she was 17, said she had flown with him to Mississippi in 1964. They walked into a hotel in Neshoba county where the FBI based itself during the investigation and he had winked at the agents. Minutes later, an agent had turned up in their room and handed Scarpa a gun.

"Greg changed his clothes - he left some money on the dresser - and he told me if he didn't come back, to take a cab to the airport and just go home."

He later told her he had kidnapped a salesman, a local klansman, catching him off guard by helping him carry a television to a car. "He put a gun in the guy's mouth. You know, he threatened the guy," she said. "He told him where the bodies were."

After Scarpa returned to the hotel, she said an agent gave him a wad of cash and took back the gun. She was surprised when she first learned he was working for the FBI. "I said, 'What do you mean, you're a rat?' And he said, 'No, I just work for them'."

Schiro is the star witness in the trial of a former FBI agent, Lindley DeVecchio, who is accused of providing confidential information that led to four murders. The prosecution claims that Scarpa gave DeVecchio money, jewellery, alcohol, prostitutes and a Cabbage Patch doll in return for inside information about informants and rivals.