In a statement, the National Archives said that on orders from President Donald Trump it had released 2,891 records related to the November 22, 1963 slaying of JFK in Dallas, Texas.
Kennedy scholars have said the documents were unlikely to contain any bombshell revelations or put to rest the rampant conspiracy theories about the assassination.
One of the documents included a transcript of a November 24, 1963 conversation with J. Edgar Hoover, who was FBI director at the time.
Hoover said the FBI informed police of a threat against the life of Lee Harvey Oswald the night before Oswald was killed. But police did not act on it, Hoover said.
File, dated April 5, 1964, details efforts to trace Lee Harvey Oswald's travel from Mexico City back to the US, released for the first time on Oct. 26, 2017 Source: AAP
The Warren Commission, which investigated the shooting of the charismatic Kennedy, 46, determined that Oswald, a former Marine sharpshooter, carried out the Kennedy assassination acting alone.
The released files are vast in number and scope, covering everything from FBI directors' memos to interviews with members of the public in Dallas who came forward trying to provide clues after that singularly unforgettable moment in US history.
Some date into the 1970s and included handwritten official notes which are hard to read.
Trump said in a memorandum he had agreed to hold back for further review some records relating to the killing.
Administration officials who requested anonymity said the majority of those requests had come from the Central Intelligence Agency and Federal Bureau of Investigation.
"Executive departments and agencies have proposed to me that certain information should continue to be redacted because of national security, law enforcement, and foreign affairs concerns," Trump said.
"I have no choice -- today -- but to accept those redactions rather than allow potentially irreversible harm to our nation's security," he said.
Trump gave agencies six months -- until April 26, 2018 -- to make their case for why the remaining documents should not be made public.
"At the end of that period, I will order the public disclosure of any information that the agencies cannot demonstrate meets the statutory standard for continued postponement of disclosure," he said.
Full and unredacted
The 2,891 records approved for release are viewable on the National Archives website, in full and unredacted form.
"The president wants to ensure that there is full transparency here," an official said, but "there does remain sensitive information in the records."
This includes, for example, the identities of informants and "activities that were conducted with the support of foreign partner organizations, either intelligence or law enforcement," the official said.
The Warren Commission's formal conclusion that Oswald killed JFK has done little to quell speculation that a more sinister plot was behind the murder of the 35th US president.
Hundreds of books and movies such as the 1991 Oliver Stone film "JFK" have fed the conspiracy industry, pointing the finger at Cold War rivals the Soviet Union or Cuba, the Mafia and even Kennedy's vice president, Lyndon Johnson.
The release of the documents is in compliance with an October 26, 1992 act of Congress which required that the assassination records held in the National Archives be released in full and unredacted 25 years later.
Kennedy assassination experts eagerly awaited the opportunity to look at the files but sought to tamp down expectations.
Gerald Posner, author of "Case Closed," which determined that Oswald did indeed act alone, said people who think the files will "have the solution to the case that everybody can settle on" are going to be disappointed.
"No one's going to abandon their belief in a conspiracy because the release of the files doesn't prove it," Posner told AFP. "They'll just say it must have been destroyed or hidden."
Experts agreed, however, that the documents may shed some light on an intriguing chapter in Oswald's life -- his trip to Mexico City about seven weeks before the slaying where he is known to have met with Cuban and Soviet spies.
The CIA and FBI may be blocking the release of certain documents to hide their own failings, said Larry Sabato, a professor of politics at the University of Virginia and the author of "The Kennedy Half Century."
"They had every indication that Oswald was a misfit and a sociopath," he said.
But neither agency informed the Secret Service, which is charged with protecting the president, he said.
Lee Harvey Oswald reacts as Dallas night club owner Jack Ruby, foreground, shoots at him in Dallas police headquarters on Nov. 24, 1963 photo. Source: DALLAS TIMES-HERALD
Oswald defected to the Soviet Union in 1959 but returned to the United States in 1962.
He was shot to death two days after killing Kennedy by a nightclub owner, Jack Ruby, as he was being transferred from the city jail.
Here's a look at what to expect:
How many files are there?
More than 3,100 documents - hundreds of thousands of pages - make up the last batch of secret files. About 30,000 redacted documents have already been released. The US National Archive will release the latest files on its website.
Will all of them be released?
No. The CIA and FBI appealed to US President Donald Trump to keep some of the records secret with Trump approving 2,800 files to be let go on Thursday night (local time). The remainings will be put under a six month review.
Why are they becoming public now?
In October 26, 1992 President George H.W. Bush signed a law requiring that all documents related to the assassination be released within 25 years, unless the president says doing so would harm intelligence, law enforcement, military operations or foreign relations. This came in part after Oliver Stone's 1991 conspiracy-theory filled film "JFK".
Will there be any bombshells?
Slim chance, according to the judge who led the independent board that reviewed and released thousands of the assassination documents in the 1990s. The files that were withheld in full were those the Assassination Records Review Board deemed "not believed relevant," Judge John Tunheim of Minnesota told The Associated Press. But Tunheim said it's possible the files contain information the board didn't realise was important two decades ago.
JFK experts believe the files will provide insight into the inner workings of the CIA and FBI. But they stress that it will take weeks to mine the documents for potentially new and interesting information.
What will the files show?
Some of the documents are related to Harvey Lee Oswald's mysterious six-day trip to Mexico City right before the assassination, scholars say. Oswald said he was visiting the Cuban and Soviet Union embassies there to get visas, but much about his time there remains unknown.
The documents contain details about the arrangements the US entered into with the Mexican government that allowed it to have close surveillance of those and other embassies, Tunheim said. Other files scholars hope will be released in full include an internal CIA document on its Mexico City station, and a report on Oswald's trip from staffers of the House committee that investigated the assassination.
