The FBI used mob muscle to solve the 1964 disappearance of three civil rights volunteers in Mississippi, a gangster’s ex-girlfriend testified Monday, becoming the first witness to repeat in open court a story that has been underworld lore for years.

Linda Schiro said that her boyfriend, Mafia tough guy Gregory Scarpa Sr., was recruited by the FBI to help find the volunteers’ bodies. She said Scarpa later told her he put a gun in a Ku Klux Klansman’s mouth and forced him to reveal the whereabouts of the victims.

The FBI has never acknowledged that Scarpa, nicknamed “The Grim Reaper,” was involved in the case. The bureau did not immediately return a call for comment Monday.

Schiro took the stand as a witness for the prosecution at the trial of former FBI agent R. Lindley DeVecchio, who is charged in state court with four counts of murder in what authorities have called one of the worst law enforcement corruption cases in U.S. history.

Prosecutors say Scarpa plied DeVecchio with cash, jewelry, liquor and prostitutes in exchange for confidential information on suspected rats and rivals in the late 1980s and early ’90s. Scarpa died behind bars in 1994.

The notion that Scarpa strong-armed a Klan member into giving up information about one of the most notorious crimes of the civil rights era has been talked about in mob circles for years.

It supposedly happened during the search for civil rights workers James Chaney, Michael Schwerner and Andrew Goodman, who were beaten and shot by a gang of Klansmen and buried in an earthen dam near Philadelphia, Miss. The case was famously dramatized in the movie “Mississippi Burning.”

Investigators struggled for answers in the early days of the case, stymied by stonewalling Klan members.

In 1994, the New York Daily News, citing unidentified federal law enforcement officials, reported that a frustrated J. Edgar Hoover turned to Scarpa to extract information. The Daily News said the New York mobster terrorized an appliance salesman and Klansman already under suspicion in the case and got him to reveal the location of the bodies.

Schiro testified Monday that she and Scarpa traveled to Mississippi in 1964 after he was recruited by the FBI. She said they walked into the hotel where the FBI had gathered during the investigation, and the gangster winked at a group of agents. She said an agent later showed up in their room and handed Scarpa a gun.

She said Scarpa helped find the volunteers’ bodies by “putting a gun in the guy’s mouth and threatening him.” She said an unidentified agent later returned to the room, gave Scarpa a wad of cash, and took back the weapon.

The killings galvanized the struggle for equality in the South and helped bring about passage of the 1964 Civil Rights Act. Seven people were convicted at the time, but none served more than six years.

Mississippi later reopened the case, winning a manslaughter conviction against former Klansmen and part-time preacher Edgar Ray Killen two years ago. He is serving a 60-year prison sentence.

Schiro’s remarks about the Mississippi episode were only a brief part of her full day of testimony.

Schiro, 62, started dating Scarpa at age 17 after meeting him in a bar. She said she had been around mobsters most of her life, so his boasts that he had been involved in 20 gangland murders didn’t frighten her.

“I was impressed,” she said.

She said she was more surprised when the Colombo crime family captain told her about his ties to the FBI. “I said, ‘What do you mean, you’re a rat?’” she recalled. “And he said, ‘No, I just work for them.’”

DeVecchio became the informant’s “handler” in 1978, and Schiro said she was allowed to sit in on weekly meetings at the couple’s apartment. She said that when Scarpa offered stolen jewelry to the agent, he took it and put it in his pocket.

Schiro testified that in the fall of 1984 she overheard DeVecchio warn Scarpa that the girlfriend of another Colombo capo was a potential “rat.”

“You know you have to take care of this?” DeVecchio said, according to Schiro.

“I’ll take care of it,” Scarpa said.

The girlfriend was gunned down at a mob social club a few days later.

Defense attorneys have sought to portray Schiro, who testified that prosecutors were paying her $2,200 a month for living expenses, as an opportunist who framed DeVecchio at the behest of overzealous prosecutors.

They have also accused her trying to improve her chances for a tell-all book deal about Scarpa.