A group of historians, including representatives of the National Coalition for History and the Society of Historians of American Foreign Relations, wrote to Mr. Leonard on Friday to express concern about the reclassification program, which they believe has blocked access to some material at the presidential libraries as well as at the archives.

Among the 50 withdrawn documents that Mr. Aid found in his own files is a 1948 memorandum on a C.I.A. scheme to float balloons over countries behind the Iron Curtain and drop propaganda leaflets. It was reclassified in 2001 even though it had been published by the State Department in 1996.

Another historian, William Burr, found a dozen documents he had copied years ago whose reclassification he considers "silly," including a 1962 telegram from George F. Kennan, then ambassador to Yugoslavia, containing an English translation of a Belgrade newspaper article on China's nuclear weapons program.

Under existing guidelines, government documents are supposed to be declassified after 25 years unless there is particular reason to keep them secret. While some of the choices made by the security reviewers at the archives are baffling, others seem guided by an old bureaucratic reflex: to cover up embarrassments, even if they occurred a half-century ago.

One reclassified document in Mr. Aid's files, for instance, gives the C.I.A.'s assessment on Oct. 12, 1950, that Chinese intervention in the Korean War was "not probable in 1950." Just two weeks later, on Oct. 27, some 300,000 Chinese troops crossed into Korea.

Mr. Aid said he believed that because of the reclassification program, some of the contents of his 22 file cabinets might technically place him in violation of the Espionage Act, a circumstance that could be shared by scores of other historians. But no effort has been made to retrieve copies of reclassified documents, and it is not clear how they all could even be located.

"It doesn't make sense to create a category of documents that are classified but that everyone already has," said Meredith Fuchs, general counsel of the National Security Archive, a research group at George Washington University. "These documents were on open shelves for years."