President Donald Trump has blocked the release of hundreds of records on the assassination of President John F. Kennedy last October, agreeing to appeals from the CIA and FBI.
Now, Mr Trump has accepted a recommendation from the National Archives to keep the lid closed, more than five decades after the 35th president was killed.
Mr Trump wrote in a memo issued today that some of the classified documents are "of such gravity that it outweighs the public interest in immediate disclosure".
The FBI and CIA had said the release could jeopardise national security.
The president ordered agencies to review the remaining documents in the next three years to determine which should be released.
Congress ordered in 1992 that all records be released in 25 years to put consistent conspiracy theories to rest.