It could have been just idle chitchat among bored prison inmates. The problem was, they weren't your average inmates, and the subject of their threatening chatter was the chief justice of the United States.

Languishing at the federal penitentiary in Lewisburg, Pa., mobsters from three top Mafia families allegedly had murder on their minds in 1979, according to recently released FBI documents. And the intended victim was Warren Burger.

At least that was the story a confidential informant told the FBI two years later. For good measure, the informant claimed, the plotters had also discussed hitting an unnamed federal judge, apparently seated in New Jersey.

What seemed to make the idea plausible were the players — big names in two of New York City's Mafia families and a Montreal don, the documents show.

The bureau took the information seriously enough that Burger was alerted. In addition, FBI headquarters in Washington approved going to mobsters in seven U.S. cities to warn them off doing anything rash that they might come to regret.

"FBI HQ concurs with Newark's recommendation to contact LCN family heads and advise them of FBI knowledge of the alleged plot in general terms," said a June 1, 1982, memo from the office of FBI Director William H. Webster. "None of the proposed victims are to be named in the contacts with the LCN figures."

"LCN" stands for La Cosa Nostra, the Italian Mafia. The memo directed FBI agents to reach out to Mafia bosses in Philadelphia; Pittsburgh; Newark, N.J.; New Haven, Conn.; New York City, Albany, N.Y., and Buffalo, N.Y.

The documents appear to reveal, for the first time, a purported Mafia plot against the chief justice of the United States. Burger was later targeted in an unrelated scheme by an unnamed Raleigh, N.C., man whose diary seemed to vaguely threaten several senior government officials.

Burger was notified of the alleged Mafia threat at the outset, as were U.S. marshals in charge of protecting him. He died in 1995 of congestive heart failure.

The FBI's 15-month investigation, which petered out when agents came up dry on evidence, was detailed in part in 143 pages of heavily redacted teletypes and other internal memos that were released in response to media Freedom of Information Act requests. They were first reported by The Gazette in Montreal.

The probe began with a Dec. 18, 1981, teletype from the FBI's Seattle office alerting headquarters and agents in five other offices of a recent conversation with "a source of information whose identity must be protected."

The topic of the two-page memo: "Plot to assassinate Chief Justice Warren Burger ... and unknown U.S. District judge." It named Phillip Rastelli, head of the Bonanno crime family in New York; Joseph Gambino, a capo in the New York family bearing his name; and Montreal mob boss Frank Cotroni as suspects.

"According to source, the subjects all have East Coast organized crime connections," the memo noted. Cotroni was further described as "the boss of the Montreal (La Cosa Nostra) family and an associate of the Bonanno LCN family."

Cotroni, Rastelli and Gambino were in Lewisburg at the time of the alleged plot — Cotroni on a drug sentence, Rastelli for racketeering, Gambino for tax evasion. Cotroni was already sprung by the time the FBI caught wind of the plan and had apparently been deported to Canada.

A fourth plotter, Joe Marino, was also named, but the FBI was unaware of any Mafia links he had, the documents show.

Months later, a new name surfaced in the plot: Anthony Provenzano, a captain in the Genovese crime family. As head of a Teamsters local in Union City, N.J., Provenzano was a suspect in the 1975 disappearance of Teamster leader Jimmy Hoffa. Hours before he vanished, Hoffa told his wife he was going to meet "Tony Pro," Provenzano's nickname.

A subsequent memo from the FBI's Newark office identified the unnamed federal judge, whose name was blacked out in the released documents, and outlined steps taken with the U.S. Marshals office there to assure his protection.

The FBI was clearly skeptical from the beginning about the information it was getting from informants, including at least one who apparently was seeking a lighter sentence in exchange for helping, the documents show. Why Burger was supposedly targeted remains unclear.

One unnamed person in the teletypes, presumably an informant, "stated that he believed the information provided to be nothing more than idle talk of frustrated inmates," according to a memo sent from the FBI's office in New York dated Dec. 30, 1981.

Nonetheless, with the safety of the nation's top jurist at stake, the investigation pushed on.

Agents from several other FBI offices — including Kansas City, Las Vegas, Los Angeles and San Francisco — were looped in to interview nearby prison inmates and provide local information that might add to the plot.

In San Francisco, agents were unable to talk to one inmate without other prisoners knowing about it because there were no interview rooms at the prison. In New Haven, agents were told by suspected Mafia members they knew nothing of the plan.

Nearly a month after Webster's June 1 directive, agents in Pittsburgh reported back that they came up empty after tracking down Sebastian John La Rocca, identified in the documents as "boss, LCN family," at his Allegheny County car wash business.

La Rocca and an associate "were apprised of the fact that the bureau was aware of an assassination plots being formulated against U.S. judges," the Pittsburgh office wrote in a June 29, 1982, memo. "La Rocca denied any knowledge of these assassination plots. La Rocca was informed by bureau agents that if an assassination or an attempt to an assassination took place, the FBI would respond with all its resources."

But little new information was uncovered as the months went on, prompting Webster in a Dec. 6, 1982, teletype to order his Seattle agents to circle back with their informant's attorney to see if he would continue to help. Three days later, Seattle agents responded by calling it "doubtful" the tipster would "cooperate further than he already has."

Webster, still in Washington, declined to comment for this story.

The end came in a March 16, 1983, teletype from the Newark office, advising that the U.S. attorney in charge of the organized crime strike force found "insufficient evidence to warrant prosecution at this time."

"No further investigation is being conducted by the Newark Division at this time," the memo advised.

Vincent Piersante, a retired organized crime investigator, said the talk about assassinating Burger and a federal judge was probably merely "a pipe dream, although some of those pipe dreams become real." Piersante spent over three decades assembling cases against organized crime figures in Michigan as a Detroit police detective and as an investigator for the state attorney general.

"When you're talking about somebody like the chief justice of the Supreme Court, that would create such a fuss that the pressure from everybody in law enforcement would be so intense that their business would suffer, and basically, they're businessmen," Piersante said. "Yes, they take care of people, but they don't want anybody to disrupt their money flow."