The US government released 2,800 previously classified JFK assassination files on Thursday, sending historians, researchers and conspiracy theorists scrambling online to scour some of the last remaining documents for fresh revelations.

But president Donald Trump delayed the release of other documents, saying he had “no choice” but to consider “national security, law enforcement and foreign affairs concerns” raised mostly by the FBI and CIA.

Conspiracy theorists – in the Oval Office and out of it – await release of JFK files Read more

The files comprise almost the final 1% of records held by the federal government and their publication follows a release in July when the record-keepers, the National Archives, posted 3,801 documents online, mostly formerly released documents with previously redacted portions. An administration official told reporters on Thursday that the files that remain secret have information that “remains sensitive depending on its context”.

Timeline JFK's assassination Show Hide

President John F Kennedy is shot dead by a sniper while his motorcade moved through Dallas. Lee Harvey Oswald, a former US marine and Soviet defector, is arrested. Oswald is shot dead by a Dallas nightclub owner, Jack Ruby. The newly sworn in president, Lyndon Johnson, orders an investigation of the assassination, named after its leader, chief justice Earl Warren. The Warren Commission determines that Oswald acted alone, without help from Cuba or Russia. The panel also finds that Ruby acted alone. A new investigation by four medical experts reaffirms the commission's finding that two bullets killed Kennedy from behind. Clay Shaw, a New Orleans businessman, is acquitted of charges of conspiracy in Kennedy's murder, after 50 minutes of deliberation. He is the only person prosecuted over the assassination. After two years of investigation, a House committee accuses previous investigators of failing to explore sufficient leads. The panel found it "likely" that conspiracies were behind Kennedy's murder, including a possible second gunman on "the grassy knoll" in Dallas. The committee also believed organized crime was "probably" involved. Oliver Stone premieres JFK, his film about the Shaw trial and the possibility of a CIA conspiracy, motivated in part to keep the US in Vietnam. A few months later, Seinfeld parodies the elaborate conspiracy theorizing around the Zapruder film and "magic bullet" theory. Congress enacts the JFK Records Collection Act and orders the release of 3,100 secret documents in an attempt to quash conspiracy theories. The documents are ordered released with a 25-year deadline. Donald Trump accuses the father of Ted Cruz of involvement in the assassination. He cites the National Enquirer, a tabloid, as his source.

Trump ordered the agencies to review those redactions over the course of six months, the official said, to ensure more documents reach the public. The next deadline for documents is 26 April 2018.

One of the first interesting documents to be unearthed as journalists, scholars and the public pored over them, was a memo written by director J Edgar Hoover that said the FBI had warning of a potential death threat to Lee Harvey Oswald.

“There is nothing further on the Oswald case except that he is dead,” Hoover wrote. “Last night we received a call in our Dallas office from a man talking in a calm voice and saying he was a member of a committee organized to kill Oswald.

“We at once notified the chief of police and he assured us Oswald would be given sufficient protection. This morning we called the chief of police again warning of the possibility of some effort against Oswald and again he assured us adequate protection would be given. However, this was not done.”

Hoover admitted that he did not have “firm” information about the man who shot Oswald dead, Jack Ruby, but nonetheless declared that his real name was Rubenstein, and noted rumors of “underworld activity”.

The FBI sent an agent to Oswald’s deathbed in the hopes of a confession, to no success. Ruby denied making any phone call.

But then Hoover said that he and Nicholas Katzenbach, the deputy attorney general, already feared the spread of rumors and conspiracy theories. He noted that Oswald had visited Mexico City, called the Cuban embassy there, and sent a letter to the Soviet embassy about a visa.

“The thing I am concerned about, and so is Mr. Katzenbach, is having something issued so that we can convince the public that Oswald is the real assassin,” Hoover said.

According to the National Archives, 88% of records related to Kennedy’s murder were already fully open and another 11% released but partially redacted. In total, that makes for about 5m pages.

After interest in conspiracy theories surged following JFK, the 1991 Oliver Stone thriller starring Kevin Costner, Congress passed the JFK Assassination Records Collection Act in 1992 and mandated that all documents must be released to the public within 25 years. That deadline was Thursday.

On Thursday, officials told reporters they would not comment on conspiracy theories. “Honestly we’re not going to comment on the content of the files,” one official said. “It’s the practice of the National Archives to leave it up to the researchers.”

In the run-up to the release, assassination experts expressed concern that agencies such as the FBI and CIA would ask Trump to block the release of information they consider to be sensitive for national security reasons, such as revealing sources, recent operations or tactics. A small number of documents are thought to have been compiled in the 1990s.

Trump tried to turn the assassination to his own political advantage last year when he spuriously claimed that the father of his then rival for the Republican presidential nomination, Ted Cruz, was seen with Lee Harvey Oswald shortly before Oswald killed John F Kennedy.

“You know, his father was with Lee Harvey Oswald prior to Oswald’s being – you know, shot,” Trump told Fox News in May last year. “I mean, the whole thing is ridiculous. What is this, right, prior to his being shot, and nobody brings it up. They don’t even talk about that. That was reported and nobody talks about it. But I think it’s horrible.”

He added: “What was he doing with Lee Harvey Oswald shortly before the death, before the shooting? It’s horrible.”

Fact-checkers have found Trump’s claimed link “implausible at best and ridiculous at worst”.

Last Saturday, Trump tweeted: “Subject to the receipt of further information, I will be allowing, as President, the long blocked and classified JFK FILES to be opened.”

Releasing the files in the face of internal pressure chimes with a narrative that Trump and his camp have previously advanced: that the president is a champion of transparency who seeks to “drain the swamp” of government agencies with their own secretive agendas who are hostile to his presidency.

Roger Stone, a Trump ally and former adviser, tweeted on Monday: “The Deep State boys will undermine the President’s order by redacting and withholding as much information as they can.”

Stone co-wrote a 2013 book positing that Lyndon Johnson, Kennedy’s vice-president and the man who succeeded him, was the mastermind behind the murder.

Last week, Stone told Alex Jones, the Infowars radio host and conspiracy theorist, that he “had the opportunity to make the case directly to the president of the United States by phone as to why I believe it is essential” that the documents be released, “because I believe they show that Oswald was trained, nurtured and put in place by the Central Intelligence Agency. It sheds very bad light on the Deep State.”

But once the thousands of new documents have been fully analysed by researchers, which will take weeks, the collection is expected to provide background information to help build a more complete picture of known events and individuals, rather than offer any stunning twists in the tale – such as a second gunman, long a favourite theory among those who do not believe Oswald shot Kennedy as the presidential motorcade travelled past the Texas School Book Depository in downtown Dallas on the afternoon of 22 November 1963.

Oswald, a former US marine who spent more than two years living in Minsk after defecting to the Soviet Union in 1959, travelled to Mexico City about two months before he killed Kennedy. Researchers are hoping the documents shed light on possible interactions with Cuban and Russian officials there. Oswald was fatally shot by Jack Ruby, a Dallas nightclub owner, two days after Kennedy’s death.

The Warren commission found in 1964 that Oswald was the lone gunman, but most Americans doubt the official line.

A Gallup poll to mark the 50th anniversary of Kennedy’s death, in 2013, found the percentage of Americans who believe that Oswald did not act alone had dipped to its lowest in nearly 50 years – even so, 61% of respondents thought that Kennedy was the victim of a conspiracy. Favourite theories included the involvement of the mafia, the federal government in general, the CIA, and Cuba under Fidel Castro.

• We’d like your help. The JFK files will be published online on Thursday here. If you’re reading through the documents and you spot an interesting fact or snippet you think we’ve missed, it would be great if you could let us know. We’ve set up a form here for contributions