A new report from the National Security Archive shows the Pentagon has recently tried to cover up historical information that was declassified and released to the public years ago.
From NSA’s report, “More Dubious Secrets”:
Pentagon classification authorities are treating classified historical documents as if they contain today’s secrets, rather than decades-old information that has not been secret for years. Today the National Security Archive posted multiple versions of the same documents—on issues ranging from the 1973 October War to anti-ballistic missiles, strategic arms control, and U.S. policy toward China—that are already declassified and in the public domain. What earlier declassification reviewers released in full, sometimes years ago, Pentagon reviewers have more recently excised, sometimes massively. The overclassification highlighted by these examples poses a major problem that should be addressed by the ongoing review of national security information policy that President Obama ordered on May 27, 2009.
For example, two different versions of a 1969 memo—the first released in 1989 and the second in 2008—show the Pentagon is trying to cover up more information now than it was twenty years ago:

The National Security Archive is an independent non-governmental research institute and library located at The George Washington University, the Archive collects and publishes declassified documents obtained through the Freedom of Information Act.