US President Donald Trump has said he will release all documents related to the 1963 assassination of President John F Kennedy, except those with the names and addresses of people who are still alive.

"After strict consultation with General Kelly, the CIA and other Agencies, I will be releasing ALL #JFKFiles other than the names and addresses of any mentioned person who is still living," Mr Trump wrote in a series of tweets, referring to his chief of staff John Kelly.

The President's tweets came days after he ordered the release of 2,800 previously secret documents, but withheld more than 300.

At the time, Mr Trump said he had "no choice" but to block the publication of the remaining documents on the recommendation of the FBI and CIA.

In the new tweets, sent on Saturday (local time), Mr Trump said he was ordering the release for "reasons of full disclosure, transparency and to put any and all conspiracy theories to rest".

The 300-plus withheld documents were due to undergo a 180-day review before a decision was made on whether to release them.

It was unclear when they would now be made public, but Mr Trump tweeted they would be released "long ahead of schedule".

Botulism pills, conspiracy theories and Lee Harvey Oswald

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 )

The records released so far have not settled the best-known, real-life whodunit in American history, but they have offered riveting details of the way intelligence services operated at the time and how they are striving to keep some particulars a secret — even now.

"The Kennedy records really are an emblem of the fight of secrecy against transparency," Peter Kornbluh, a senior analyst at the private National Security Archive research group in Washington, said.

"The 'secureaucrats' managed to withhold key documents and keep this long saga of secrecy going."

Historians, journalists and amateur sleuths have spent the days since their release poring through the documents, uncovering details about botulism pills, conspiracy theories, what the government might have known and still will not say about Lee Harvey Oswald.

Here are some of the tidbits unearthed so far:

Hoover worried about fallout from JFK's death

Just a few hours after Lee Harvey Oswald was killed in Dallas, FBI director J Edgar Hoover dictated a memo saying the government needed to issue something "so we can convince the public" that Oswald killed president John F Kennedy.

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The memo was in the latest trove of Kennedy assassination files released last week. The FBI director composed the memo on November 24, 1963 — two days after Kennedy was killed and just hours after nightclub owner Jack Ruby fatally shot Oswald in the basement of the Dallas police station.

Hoover said the FBI had an agent at the hospital in hopes of getting a confession from Oswald, but Oswald died before that could happen. Hoover said he and a deputy were concerned about "having something issued so we can convince the public that Oswald is the real assassin".

Hoover laments how Kennedy's successor Lyndon B Johnson was considering appointing a presidential commission to investigate the assassination. Hoover said he suggested the FBI give an investigative report to the attorney-general complete with photographs, laboratory work and other evidence. That report, he thought, could be given to Johnson and he could decide whether to make it public.

"I felt this was better because there are several aspects which would complicate our foreign relations," Hoover wrote. He said Oswald wrote a letter to the Soviet Embassy in Washington, which the FBI intercepted, read and resealed. Hoover said the letter had been addressed to the Soviet Embassy official "in charge of assassinations and similar activities on the part of the Soviet government. To have that drawn into a public hearing would muddy the waters internationally," Hoover wrote.

Besides, Hoover said, the letter was unrelated proof that Oswald committed the murder.

LBJ thought assassination was payback for JFK

Everyone has their theories, and so did president Lyndon B Johnson. Johnson believed Kennedy was behind the assassination of the South Vietnamese president weeks before his death and that Kennedy's murder was payback, the newly released documents say.

US director of central intelligence Richard Helms said in a 1975 deposition that Johnson "used to go around saying that the reason (Kennedy) was assassinated was that he had assassinated president [Ngo Dinh] Diem and this was just justice".

"Where he got this idea from I don't know," Helms said in the deposition.

Diem and his brother were killed on November 2, 1963 after a coup by South Vietnamese generals.

This isn't the first time Johnson's theory has been aired. He was also quoted in Max Holland's book, The Kennedy Assassination Tapes, as saying that Kenney died because of "divine retribution".

"He murdered Diem and then he got it himself," Johnson reportedly said.

KGB put spotlight on LBJ

The former Soviet Union's intelligence agency allegedly claimed it had information tying Johnson to the Kennedy assassination.

In a 1966 letter to a presidential assistant, Hoover wrote that an FBI source reported KGB officials claimed to have information in 1965 "purporting to indicate" Johnson had a role in the assassination.

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

The source had "furnished reliable information in the past," the letter states. The source said the KGB residency in New York received instructions from Moscow in September 1965 to "develop all possible information" on Johnson, who was considered "practically an unknown" to the Soviet government at the time.

Those instructions contained the assertion that the KGB had information tying Johnson to an assassination plot, according to the source.

Johnson has long been a focus of some conspiracy theorists, but no credible information has been revealed linking him to the assassination.

Castro, the CIA and the mobster's mistress

A 1975 document described the CIA's US$150,000 offer to have Cuban leader Fidel Castro assassinated — but the mob insisted on taking the job for free.

The murder contract was detailed in a summary of a May 1962 CIA briefing for then-attorney-general Robert Kennedy. By then, the Kennedy White House had launched its unsuccessful Bay of Pigs invasion of Cuba and several assassination attempts against Castro had failed.

At least two efforts to kill Castro were made with CIA-supplied lethal pills and organised crime muscle in early 1961, according to the document. The CIA's mob contacts included John Rosselli, a top lieutenant to Chicago mob boss Sam Giancana, who weren't told but guessed the CIA was behind the offer. The pair, later victims of mob hits, said they wanted no part of any payment but US$11,000 in payments were made for expenses.

The mobsters came to the attention of the CIA a year earlier when Giancana asked a CIA intermediary to arrange for putting a listening device in the Las Vegas room of an entertainer he suspected of having an affair with Giancana's mistress. The task was handed off to a private investigator named Arthur Balletti, who put the listening device in a phone in the hotel room.

AP/Reuters