US President Donald Trump is delaying the release of some files related to the John F Kennedy assassination that were due to come out on Thursday.
Another 2,800 records have been approved for release.
White House officials say Trump said in a memo that he had "no choice" but to keep others secret because of national security concerns.
He will have those records further reviewed for the next six months.
Officials say Trump will impress upon federal agencies that JFK files should stay secret after the six-month review "only in the rarest cases".
Congress mandated in 1992 that all assassination documents be released within 25 years, but Mr Trump has the power to block them on the grounds that making them public would harm intelligence or military operations, law enforcement or foreign relations.
The trove due to be released on Thursday was expected to include more than 3,000 documents that have never been seen by the public and more than 30,000 that have been previously released but with redactions.
Much of Thursday passed with nothing from the White House or National Archives except silence, leaving it unclear how the government would comply with a law requiring the records to come out by the end of the day - unless Trump had been persuaded by intelligence agencies to hold some back.
White House officials said the FBI and CIA made the most requests within the government to withhold some information.
No blockbusters had been expected in the last trove of secret files regarding Kennedy's assassination on November 22 1963, given a statement months ago by the Archives that it assumed the records, then under preparation, would be "tangential" to what's known about the killing.
But for historians, it was a chance to answer lingering questions, put some unfounded conspiracy theories to rest, perhaps give life to other theories - or none of that, if the material adds little to the record.
Researchers were frustrated by the uncertainty that surrounded the release for much of the day.
"The government has had 25 years with a known end-date to prepare #JFKfiles for release," University of Virginia historian Larry Sabato tweeted in the afternoon. "Deadline is here. Chaos."
Asked what he meant, Sabato emailed to say: "Contradictory signals were given all day. Trump's tweets led us to believe that disclosure was ready to go. Everybody outside government was ready to move quickly."
Trump had been a bit coy about the scheduled release on the eve of it, tweeting: "The long anticipated release of the #JFKFiles will take place tomorrow. So interesting!"
Experts say the publication of the last trove of evidence could help allay suspicions of a conspiracy - at least for some.
"As long as the government is withholding documents like these, it's going to fuel suspicion that there is a smoking gun out there about the Kennedy assassination," said Patrick Maney, a presidential historian at Boston College.
Experts said intelligence agencies pushed Trump to keep some of the remaining materials secret - the CIA didn't comment on that.
Whatever details are released, they are not expected to give a definitive answer to a question that still lingers for some: Whether anyone other than Lee Harvey Oswald was involved in the assassination.
The Warren Commission in 1964 reported that Oswald had been the lone gunman, and another congressional probe in 1979 found no evidence to support the theory that the CIA had been involved. But other interpretations, some more creative than others, have persisted.