The documents have either never been disclosed or been made public only in redacted form, and are due to be released by the National Archives and Records Administration on Thursday under a law passed in 1992 after the Oliver Stone movie “JFK” stoked interest in Kennedy-related conspiracies. The last of the documents were required to be released 25 years after the law was signed, but the incumbent president, in this case Mr. Trump, can order some withheld in response to concerns by the intelligence agencies. White House officials said he had not made up his mind whether to do so.
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Historians and conspiracy investigators are eager to see what the documents may yet reveal about Lee Harvey Oswald and any ties he may have had to the Cubans, Soviets, C.I.A., F.B.I. or mafia. Some hope for a better understanding of Oswald’s trip to Mexico City, where he visited the Cuban Consulate in the weeks before the assassination at Dealey Plaza in Dallas on Nov. 22, 1963.
Still, some specialists on the killing warned against expecting any stunning revelation. “I don’t think it will turn the case on its head,” said Gerald Posner, author of “Case Closed,” the 1993 book that concluded that Oswald indeed killed Kennedy on his own.
“We’re not going to find some secret memo from J. Edgar Hoover drawing out the escape path for Lee Harvey Oswald,” he said. “The public expectations are very high — they’ve heard about secret files, they know they’ve been locked up for all these years. The average person may think there’s a bombshell in there.”
But Mr. Posner said the files might draw a fuller picture of the early 1960s beyond the specific questions about the assassination. “This is all about the Cold War and spooks and spies and Mexico City,” he said. “This is about a time when we know the government was in league with the mob to kill Castro. Cold War scholars and historians may find this as interesting as Kennedy assassination researchers.”
According to the archives, 88 percent of the documents in the collection created by the 1992 law have been released in full and another 11 percent have been released with portions redacted. Just 1 percent have been withheld in full until now. Most have remained secret because they were declared “not assassination related” or “not believed relevant.” Officials said many of those were documents created as late as the 1990s to describe how intelligence collection worked.
Jefferson Morley, an author who spent years suing the C.I.A. for documents related to the Kennedy. assassination, said he thought it likely that Mr. Trump would defer to some agency demands and withhold a portion of the archive. But he said he nonetheless hoped it would answer some questions for researchers that linger after nearly 54 years.
“There won’t be any smoking gun,” said Mr. Morley, editor of the assassination website JFKfacts.org, who re-examined the period for his new book, “The Ghost: The Secret Life of C.I.A. Spymaster James Jesus Angleton.” “But it will fill in the picture of the pre-assassination surveillance of Oswald,” especially during his visit to the Cuban Consulate in Mexico City.
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Mr. Morley said that the C.I.A. and the F.B.I. were well aware of Oswald, a former defector to the Soviet Union, before the killing. “The idea that Oswald came out of the blue and shot the president is false,” he said. “The C.I.A. had a deep file on him.”
Mr. Morley also said that with the potential release of what may total more than 100,000 pages, no one should expect instant answers on what they contained. “There will be good stuff in there, but you’re not going to find it in the first two hours,” he said.
Max Holland, a Washington writer and author of the 2004 book “The Kennedy Assassination Tapes,” said he believed expectations about potential revelations from the files were overblown.
He noted that while the documents have not been previously made public, they all were seen years ago by the J.F.K. Assassination Records Review Board and were unlikely to significantly affect the official story. He cautioned against conspiratorial thinking that runs against the evidence, which he finds persuasive, that Oswald alone killed Kennedy.
“I can understand why people are curious,” Mr. Holland said. “But the level of distrust in this country is such that people will believe anything. The problem is really with us.”
Indeed, the Kennedy assassination has continued to intrigue and puzzle the American public long after most of the main players have died. While the Warren Commission concluded that Oswald acted alone, most people have never accepted the official version of events. A poll by Gallup in 2013, at the time of the 50th anniversary, found that 61 percent of Americans still believed that others besides Oswald were involved — and that was the lowest percentage of skeptics found in nearly a half century.
“We just have to realize that there is never going to be an explanation of the Kennedy assassination that will satisfy everyone,” Mr. Beschloss said. “That will never happen. At the same time, there are still mysteries on which these files might shed some light.”