United States President Donald Trump has ordered the declassification and release of all remaining files related to the assassination of former US President John F Kennedy, the focus of popular conspiracy theories for six decades.

Trump?s executive order signed on Thursday also calls for the release of the last remaining records on the assassinations of Robert F Kennedy, JFK?s younger brother, and the civil rights icon Martin Luther King Jr.

?This is a big one. A lot of people have been waiting for this for years, for decades,? Trump said as he signed the order at the White House.

?And everything will be revealed.?

Under Trump?s order, the Director of National Intelligence must present a plan within 15 days for the ?full and complete release? of files related to JFK?s assassination and a plan within 45 days for the release of documents on the other two assassinations.

The circumstances of JFK?s assassination in Dallas, Texas, on November 22, 1963, have transfixed Americans for decades, with surveys showing widespread doubt about official explanations of the killing.

In a 2023 Gallup poll, 65 percent of Americans said they did not believe the Warren Commission?s finding that Lee Harvey Oswald, a US Marine veteran arrested over JFK?s death, acted alone in killing the president.

Twenty percent of respondents said they believed Oswald conspired with the US government, while 16 percent said they believed that he worked with the CIA.

Robert F Kennedy Jr, Trump?s nominee for health secretary and the son of Robert F Kennedy, claimed in a 2023 interview that there was ?overwhelming? evidence of CIA involvement in his uncle?s killing and ?very convincing? but ?circumstantial? evidence that the intelligence agency was involved in his father?s death.

After signing his order at the Oval Office, Trump handed the pen he used to an aide, saying, ?Give that to RFK Jr?.

Criticising Trump?s order, Jack Schlossberg, the grandson of JFK, said his grandfather?s death had not been part of an ?inevitable grand scheme.?

?Declassification is using JFK as a political prop, when he?s not here to punch back. There?s nothing heroic about it,? Schlossberg, who works as a political correspondent for Vogue magazine, said in a post on X.

In 1992, the US Congress passed a law mandating that outstanding files related to the JFK assassination be released within 25 years unless the president determined that the harm to national security outweighed the public interest in disclosure.

Trump ordered the release of more than 2,800 documents upon the arrival of the 2017 deadline but bowed to pressure from the CIA and FBI to withhold thousands of more files pending review.

The administration of former US President Joe Biden ordered the release of approximately 17,000 more documents, leaving fewer than 4,700 withheld in part or in full.

In total, more than 99 percent of some 320,000 documents reviewed since the passage of the 1992 law have been released, according to The National Archives.

King, whose ?I Have a Dream? speech became a defining moment of Black Americans? struggle for equality, was fatally shot outside a motel in Memphis, Tennessee, on April 4, 1968.

Robert F Kennedy was shot dead at a Los Angeles hotel on June 5, 1968, shortly after wrapping up a speech to mark his victory in California?s Democratic presidential primary. (AFP)