The two reputed Mafia leaders were driving through New Orleans together some 12 years ago when their car radio barked out the latest item about District Attorney Jim Garrison's investigation into the assassination of President Kennedy.

Santo Trafficante listened for a moment and then, according to an April 11, 1967, FBI report, turned to Carlos Marcello, who was driving.

"Carlos," he said, "the next thing you know they'll be blaming the assasination on us."

The final report of the House Assasinations committee doesn't go quite that far, but it comes close. After suggesting the possibility of a plot against Kennedy by individual gangland leaders, it labels Marcello and Trafficante "the most likely family bosses of orgainzed crime to have participated in such a scheme -- although the ambivalent report then goes on to call the notion "unlikely."

Additional details (but not the April 11, 1979, FBI report which was obtained from another source), are contained in House committee staff studies scattered through 27 volumes of backup material published this week.

The FBI, according to the House investigators, was informed years ago of an assassination threat that Marcello allegedly made in September 1962. But instead of investigating the report, the House study said the bureau tried to discredit the individual who made it, a private investgator named Edward N. Becker.

Reiterating his story for the comittee in an interview last fall, Becker said that Marcello had been quite angry and "clearly stated that he was going to have President Kennedy murdered in some way."

"Beckert," the House staff report continued, "commented that Marcello had made some kind reference to President Kennedy's being a dog and Attorney General Robert Kennedy the dog's tail, and had sad "the dog will keep biting you if you only cut off its tail,' but that if the dog's head were cut off, the dog would die."

The FBI first learned of the alleged episode in early May 1967, in the midst of the Garrison investigation, when a writer on organized crime named Ed Reid showed one of his book manuscripts to Los Angeles FBI agents, apparently as part of an effort to trade information. Reid, however, was under the impression that his source, Becker, has told the FBI about it back in 1962.

Becker, however, had not done so, telling House investigators that he "would have been afraid" to repeat Marcello's remarks to anyone during that period, out of concern that Marcello or his associates might learn" of it.

The FBI evidently decided that Becker's information was "greatly undermined" by his failure to speak up sooner and never even interviewed him. Instead, according to the House study, the bureau -- relying on derogatory information about Becker from sources such as reputed underworld associate Sidney Korshak -- dismissed Becker as "unreliable" and even tried to discredit his story about Marcello.

In fact, the House investigators said, two efforts, one involving a personal visit by a Los Angeles FBI agent, were made to convince Reid of Becker's unreliability in hopes that "the Carlos Marcello incident would be deleted" from Reid's forthcoming book.

Reid mentioned it anyway, in a book called "The Grim Reapers," published in 1969. He quoted Marcello once summarily deported to Guatemala at Robert Kennedy's orders, as having shouted an old Sicilian threat, "'Livarsi na petra did la scarpa' -- 'Take the stone out of my shoe'" against the Kennedy brothers.

According to Reid's account, which Becker described later as "basically correct," Marcello also spoke of "using 'a nut' to do the job."

Now 57, Becker told House investigators in his interview with them last fall that his story "is truthful. It was then and it is now. I was there." He protested that "the FBI -- their agents in Los Angeles -- have tried to discredit me. They've done everything expect investigate the information I gave Reid. They apparently have always said it was not the truth, but they've never investigated it to arrive at that judgement."

The meeting with Marcello assertedly took place at his Churchill Farms estate near New Orleans. Becker said he had gone there several times with Marcello's nephew, a Shreveport oil geologist named Carl Roppolo, to try to get Marcello's support for a proposed business venture of theirs.

According to the House staff report, Becker recalled "that Marcello had made his remark about the Kennedy brothers after Becker said something to the effect that 'Bobby Kennedy is really giving you a rough time.'

Although he was a reputation for being exceedingly secretive, Marcello seems strangely talkative, by Becker's account. To House investigators, "Becker stated that Marcello also made some kind of reference to the way in which he wanted to arrange the president's murder. Marcello 'clearly indicated' that his own lieutants must not be indentified as the assassins, and that there would thus be a necissity to have them use or manipulate someone else to carry out the actual crime."

Testifying under a grant of immunity, Marcello was questioned about the allegations in Reid's book in an executive sessions before the House committee on January 11, 1978. He denied that the meeting or the discussion ever took place.

Summed up the House staff report:

"Marcello stated that Churchill Farms was not a place where he would conduct a meeting; that the estate was only used for hunting and was the location of various duck blinds. Marcello further testified that he did not have to discuss his deporation with associates because "Everybody in the United States knowed I was kidnaped. I didn't have to discuss it . . . I told the whole world it was unfair. Anybody who taked to me said it was unfair.'

"When asked if he had ever made any threat against Attorney General Kennedy or had spoken of taking any physical action against him, Marcello stated, 'No sir; I never said anything like that.' When asked if he had ever spoken of taking such action against President Kennedy or had threatened him in any way, Marcello stated, 'Positively not, never said anything like that,'"

In its final report, the House assassinations committee was sharply critical of the FBI's handling of Becker's allegations, and said it had been able to determined that he had been able "in fact associated with various underworld figures, including at least one person well-acquanited with the Marcello organization."

The committee added, however, that as a consequence of Becker's underworld involvement, Becker "had a questionable reputation for honesty and may not be a credible source of information."

The House members added that it was "unlikely that an organized crime leader personally involved in an assassination plot would discuss it with anyone other than his closest lieutenants, although he might be willing to discuss it more freely prior to a serious decision to undertake such an act."