The FBI yesterday made public thousands of new documents that reveal intense backstage bickering and infighting over how to investigate President Kennedy's assassination, but apparently no startling new evidence about the murder.

At one point, nearly three weeks after the President was killed in Dallas on Nov. 22, 1963, then FBI Director J. Edgar Hoover professed far greater misgivings than he ever publicly acknowledged about the possibility that others besides Lee Harvey Oswald my have been involved.

Recounting a conversation he had just had with Warren Commission general counsel Lee Rankin on Dec. 12, 1963, Hoover said in a four-page memo that morning that "I personally believe Oswald was the assassin" but "the second aspect as to whether he was the only man gives me great concern . . . "

But it remained unclear whether the canny FBI director was voicing this fear simply for the record or whether he was really troubled by it. As evidence of his apprehensions, he said he told Rankin of "several letters" written to Oswald from Cuba "referring to the job he was going to do . . ."

In another memo written that day, Hoover, however, dwelt on the suspicious nature of those letters, noted that they had been written on the same typewriter although ostensibly signed by different individuals, and concluded that they were apparently "an attempted hoax," Rankin was evidently told none of this.

The massive compilation of documents, a total of 40,001 pages released by the FBI under the Freedom of Information Act, is full of similar puzzles and contradictions on the events following the Kennedy murder.

Another 40,000 pages is expected to be released next month as a result of the bureau's "Project Onslaught," a four-month, $2.8 million effort to reduce its huge backing of FOIA requests. FBI officials said the release of the Kennedy papers alone, covering more than 200 volumes, cost, by conservative estimate, at least $188,000.

Many of the papers released yesterday had been previously made public over the years, but these were re-released together with the new documents without any distinction. Uncounted pages bearing on the Kennedy assassination in key FBI field offices, such as Dallas and New Orleans, are still locked up. The files being released consist solely of FBI headquarters documents in three categories: teh JFK assassination, Lee Harvey Oswald and Jack Ruby.

Just three days after the assassination, high-ranking FBI officials began already to reflect in theire internal correspondence the frustration of trying to prove, in effect, a negative proposition - that a foreign conspiracy did not exist.

Courtney Evans, a top aid eto Hoover, mused about a telephone conversation he had with then Deputy Attorney General Nicholas B. Katsenbach, in which the two discussed the pending bureau report and the pressures to make the findings of the investigation public.

"We are being called upon, in many instances, to prove the negative. Katzenbach notes it is more difficult to prove that something did not happen than it is to prove that it did happen.

"From the facts disclosed in our investigation, there is no question that we can submit in our report convincing evidence beyond any doubt showing Oswald was the man who killed President Kennedy," Evans said. But, he allowed. "We must be factual and recognize that a matter of this magnitude cannot be fully investigated in a week's time."

A handwritten notation - the writer unknown - added, "Just how long do you estimate it will take. It seems to me we have the basic facts now."

The next day, Evans wrote another memo to Hoover aide Alan Belmont repeating not only Katzenbach's concern for a speedy report of the FBI's findings, but also mounting worries about press speculation about a conspiracy.

One of the dangers Katzenbach cited, according to this memo, was that an inquiry by state officials in Texas would delve into the sensitive area of Oswald's motives.

"One of the dangers which Katzenbach sees is the possibility that the state hearing . . . may develop some pertinent information not now known," Evans wrote.

In an effort to minimize that danger, he said, he dispatched a Justice Department official to talk with Texas officials "to have them restrict their hearing to the proposition of showing merely that Oswald killed the President . . . He hopes to avoid the state hearing going into the question of motive or trying to resolve the communist angle."

The same memo revealed that Katzenbach had learned, "on an extremely confidential basis," that Washington attorney Abe Fortas, a confidant of President Johnson, had talked with Johnson and argued against the idea of having a presidential commission look into the assassination.

Fortas' argument, according to Evans, was that even to announce such a commission would suggest there was more to the assassination than one man acting alone, and that it would also call into question teh effectiveness of the FBI.

"Fortas, of course, is no friend of the bureau and there would appear to be some obvious underhanded motive in his using us in his argument, although we don't know what this is," the Hoove aide said.

The Evans remark ran counter to a widespread assumption that Fortas was a principal advocate of the creation of the Warren Commission.

Signs of internal rivalry and occasional bickering among the highest levels of the assassination probe began to develop just a few weeks after the President was slain.

For instance on Dec. 2, Katzenbacj telephoned Evans, according to a memorandum of conversation, and said that Chief Justice Earl Warren was "strongly opposing" any publication of the still incomplete FBI report on the assassination. Warren felt so strongly about it, Katzenbach is quoted as saying, that he would resign from the commission if anything was released.

The memo went on to say that Katzenbach also regarded Warren Olney, who was being considered as chief counsel to the commission, as "undesirable" for the post. His reasons were not spelled out.

The next day, Cartha (Deke) DeLoach, assistant FBI director, reported in a memo that Kazenbach had called hin and said he believed that Olney had "definitely been dumped," and that he had given the go-ahead for the appointment of Lee Rankin as general counsel.

Occasionally testy discussions also developed at the Justice Department and the White House over the form of a press release that was to be issued concerning the investigation.

According to a Dec. 9 DeLoach memo, Hoover felt "strongly" that only the first paragraph of a proposed press release - asserting blandly that an FBI report had been transmitted to the comission - should be released.

However, Katzenbach is quoted as saying there was a "statemate" at the White House in connection with the release, with Fortas arguing that the entire release should be issued.

Fortas' argument, DeLoach wrote, was that "speculation, rumors and gossip should be dispelled right now.

"He claimed that it made no difference that the presidential commission would present findings to the President and the general public. He felt rumors concerning collusion between Ruby and Oswald also concerning international conspiracy, should be hit hard now," DeLoach wrote.

As a result of the bickering, he said, there would be no press release. Instead, Katzenbach was to write a letter to Warren and each commission member appealing for a quick report so that rumors and speculation could be eliminated.

Referring to Katzenbach's letter, DeLoach said, "He goes on to point out that (the investigation thus far has nailed down the point that Oswald assassinated the President and had no assistance. He further mentions that Ruby operated strictly as an "individual and had no assistance. He points out that there is no evidence of collusion or international intrigue."

Interestingly, the suppressed press release did not seem worthy of so much bureaucratic anguish.

It merely stated that an FBI report had been sent tot he commission, and that the Justice Department was of the opinion that scientific ebamination of evidence had established that Oswald shot Kennedy. It also said that no evidence had been uncovered to date of a conspiracy involving and persons, including Jack Ruby, but that this aspect of the probe would be investigated further.

The letters Hoover cited to Rankin as potential indicators of a conspiracy pointing toward Cuban Premier Fidel Castro concerned a so-called Pedro or Peter Charles. Hoover said he told Rankin that they alluded to Oswald's "good marksmanship" and stated "when it was all over, he (Oswald) would be brought back to Cuba and presented to the chief." Hoover said it was not certain that "the chief" was Castro, but suggested that the letters were sufficient reason for his strongly urging Rankin at that point in the inquiry "that we not reach conclusion Oswald was the only man."

Despite all that, the FBI laboratory had by then made a thorough examination of the Charles letter, which was dated Nov. 10, 1963, but not postmarked in Havana until Nov. 28, 1963, and compared it with another letter, also postmarked in Havana on Nov. 28, 1963, and addressed to Attorney General Robert F. Kennedy.

The first letter, from Charles to "Friend Lee," was intercepted in Dallas Dec. 5, 1963, and was not , in itself, a new disclosure. It was written in such a way as to suggest that Oswald had been paid off by Charles in Miami to carry out an unspecified mission that involved accurate shooting.

The other letter, dated Nov. 27, 1963, and addressed to Robert Kennedy by someone calling himself "Mario del Rosario Molina," asserted that Oswald had killed the President at the direction of Pedro Charles, a Castro agent who was supposed to have met Oswald in Miami some two months earlier and paid him $7,000.

The FBI's Intelligence Division reported on Dec. 11, 1963, however, that both letters had been written on the same Remington typewriter, that the same type pen and ink had been used to sign both letters, and that the two postmark contained the same irregularities.

" . . . (It) seems clear that this matter represents an attempted hoax, possibly perpetrated by some anti-Castro group seeking to attach blame to the Castro government," the FBI's W. R. Wannall informed his boss in the Intelligence Division, William C. Sullivan.

Hoover indicated the next day, in one of the newly disclosed memos, that he agreed, saying that "it appears this matter represents an attempted hoax," especially in view of the delayed mailing, but adding that the CIA and State department had been asked to be alert for any signs that Molina might try to get to Venezuela as his Nov. 27 letter had stated.

There was no indication in the documents reviewed yesterday that he ever did.

Rich in detail, the documents portray a thoroughness which seems to transcend even the FBI's reputation for attention to detail. No matter how obscure or ambiguous, each lead was tracked down with personal interviews, tireless background checks and a semming obsession for completeness.

The Warren Commission encouraged such thoroughness, as Rankin wrote increasing numbers of letters to the FBI telling it about new tips received or claims of overheard conversations.

At one point, FBI officials seemed to show minor annoyance with what the commission was forwarding to the bureau.

"From general appearances of this communication, it would appear the correspondent has little, if any information . . . and might also be a mental case," the FBI said of one letter-writer. "However, in view of the commission's request, it is not felt we have any alternative but to handle the request," the memo added.

No matter who was brought to the bureau's attention - or no matter for what purpose - the subject would be checked out. Somebody at American Broadcasting Co. sent Hoover a complimentary record album with key broadcasts about the Dallas tragedy, and enclosed a form letter saying it was from ABC-Paramount Theaters President Leonard H. Goldenson. Hoover wrote a cordial letter of thanks, to which a bureau offical attached the note, "Bufiles (bureau files) contain no derogatory information regarding Mr. Goldenson.