The deadline is looming for the release of hundreds of thousands of pages of documents on the assassination of John Fitzgerald Kennedy and experts say it could expose a CIA cover-up.

The Warren Commission established after Kennedy's death concluded Lee Harvey Oswald was the sole gunman.

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Many credible experts agree that is unlikely to be challenged by the new material, but claims from intelligence agencies that they knew little of Oswald before the assassination may face renewed scrutiny.

"I mean this is the event that really changed America in so many ways. The only other comparison is 9/11 but this is the event that really made Americans believe that they couldn't trust their government and they couldn't expect their government to tell them the truth," former New York Times correspondent and author of A Cruel and Shocking Act, Philip Shenon said.

'CIA and the FBI knew a lot about Oswald'

JFK and Jackie in the limousine as it makes its way through Dallas on November 22, 1963. ( Walt Cisco/Dallas Morning News )

Even close to 55 years after that shocking day in Dallas, the thirst for new information is as strong as ever.

The release tomorrow is expected to include more than 3,000 never before seen documents as well as previously redacted material.

There is not expected to be a smoking gun revealing a second shooter beyond Oswald on the sixth floor of the book depository, but there could be other revealing documents.

"I think the evidence is pretty clear that Oswald was the only shooter. What I don't eliminate is the possibility he was encouraged to do it, was aided to do it or at least that he told other people he was going to do it," Professor Larry Sabado of the University of Virginia said.

Oswald was a former marine who had defected to the Soviet Union before returning to the United States, and intelligence agencies' claims about him have been disputed for generations.

"Immediately after the assassination, the CIA and the FBI wanted desperately to portray Oswald as a lone wolf, a guy they never could've stopped," Mr Shenon said.

"Well, the truth seems to be that actually the CIA and the FBI knew a lot about Oswald and if they just acted on the information in their own files then the assassination would've been prevented, and I think we're going to see a lot more information along those lines."

Oswald's visit to Cuban consulate and Soviet embassy

Lee Harvey Oswald as he is shot by Jack Ruby in the basement of Dallas Police Headquarters on November 24, 1963. ( Ira Jefferson Beers Jr/Dallas Morning News )

Of particular interest is a trip Oswald took to Mexico City weeks before the shooting in which he visited the Cuban consulate and Soviet embassy.

A known defector visiting what were at the height of the Cold War some of the most watched diplomatic outposts in the world would surely have been of interest to the local CIA spy chief.

"The CIA told the Warren Commission, 'We didn't really know much about this guy, he passed through and we weren't really paying attention'," former Washington Post editor and author of The Ghost, Jefferson Morley said.

"That's another story that's basically false. They were paying very close attention to Oswald six weeks before the assassination and I think we'll find out more about the nature of that interest, what they learned, what was reported."

The question of whether that intelligence could have saved an American president has never been satisfactorily answered for those that make a career out of this dark chapter in America's history.

"The question in my mind has always been, doesn't the CIA know that this man Oswald might be a threat and isn't it possible that they bungled that intelligence, they missed their chance to save Kennedy's life?" Mr Shenon said.

One of the more fanciful conspiracy theories

Lyndon Johnson is sworn in aboard Air Force One, hours after the assassination of JFK. ( Cecil W. Stoughton )

The release of these documents was spurred by the blockbuster 1991 movie JFK, directed by Oliver Stone.

The movie is filled with loose allegations and absurd connections but it prompted a public outcry which led to Congress agreeing to release the secret material by October 2017.

US President Donald Trump said he would not block the JFK files, but many experts worry that some of the most sensitive material on the assassination of America's first and only Catholic president will still be redacted.

It will take days and weeks to make sense of the huge document dump.

"You know the conspiracy theories you hear about the Mafia or space aliens or whatever — for most of them there's absolutely no evidence to support them," Mr Shenon said.

"There is evidence to support the idea though that other people knew what Oswald was going to do and may have encouraged him to do what he was going to do and by definition that is a conspiracy."

One of the most fanciful theories — definitely in the tinfoil hat category but popular among a small group on both the left and right — is that then-Vice President Lyndon Johnson (better known as LBJ) was in cahoots with intelligence agencies in ordering the kill.

Don't hold your breath for revealing documents on that front.

"The LBJ done-it theory, I mean it's pervasive, it's especially strong in Texas — the evidentiary basis of it is very slight," Mr Morley said.

"There's nothing to connect Johnson to Oswald. Certainly, not in the way that there's the ability to connect the CIA to Oswald or the Cuban Intelligence to Oswald.

"So the LBJ conspiracy theory is used, I think, as a way of demonising and discrediting American liberalism because Johnson was a hero of American liberalism in the 1960s.

"But the LBJ done-it theory does not have a lot of support among serious historians of the assassination."

Was CIA protecting itself?

Jackie Kennedy leads the funeral procession for JFK from the White House to the Cathedral of St Matthew the Apostle on November 25, 1963. ( Robert Knudsen/White House )

Mr Morley hopes it will reveal new information about spymaster James Angleton, who he wrote his book The Ghost about.

He believes the CIA covered up their knowledge of Oswald to protect themselves.

"He would've lost his job, he would've been fired instantly, and other people at the CIA would've been fired too," Mr Morley said.

"And that's why they covered up. Were they covering up a manipulation of Oswald, an operation to use him, or were they covering up their own incompetence? This guy came through, they were paying close attention to him and he up and shot the president.

"Those are the two possibilities and the information that we're going to get, if we get all of it, may shed real new light on this."