US intelligence agencies have been removing thousands of historical documents from public access, the New York Times has reported.
The reclassification is said to have accelerated under President Bush
The restoration of classified status to more than 55,000 pages began in 1999, the paper said.
At that time, the CIA and five other agencies reportedly objected to what they saw as a "hasty release" of sensitive information.
The files include documents already published or obtained by historians.
The New York Times said the reclassification programme accelerated after President Bush took office and especially after the 9/11 attacks.
But because it runs in secrecy, it continued without being noticed until December 2005.
According to the report, it was intelligence historian Matthew Aid who noticed that dozens of documents he had copied years ago had been withdrawn from the National Archives' open shelves.
Those are said to include decades-old State Department reports from the Korean War and the early Cold War.
Under existing guidelines, government documents are supposed to be declassified after 25 years unless there is a particular reason to keep them secret.
But some historians argued that the reclassification program is removing material that can do no conceivable harm to national security, the New York Times said.
Mr Aid mentioned among the "innocuous" files removed a 1948 memorandum on a CIA scheme to float balloons over countries behind the Iron Curtain to drop propaganda leaflets.
Another reclassified document that Mr Aid had copied gives the CIA's assessment on 12 October 1950 that Chinese intervention in the Korean War was "not probable in 1950" - two weeks before Chinese troops crossed into Korea.
Another historian, William Burr, is quoted by the New York Times as saying that he considered "silly" the reclassification of a dozen files he obtained at the National Archives.
He mentioned a 1962 telegram from the then US ambassador to Yugoslavia containing an English translation of a Belgrade newspaper article on China's nuclear weapons program.
After Mr Aid and other historians complained, the archives' Information Security Oversight Office began an audit of the reclassification program, according to the New York Times.