Records about the US Navy Seals' raid on Osama bin Laden's hideout were ordered to be purged from Pentagon computers and sent to the CIA - where they could be more easily shielded from ever being made public.
A draft report by the Pentagon's inspector-general briefly described the secret move, which was directed by the top US special operations commander, Admiral William McRaven.
The transfer did not set off alarms within the Obama administration even though it appears to have sidestepped rules governing federal records and circumvented the Freedom of Information Act.
President Barack Obama has pledged to make his administration the most transparent in US history.
The CIA said the documents were handled in a manner consistent with the fact that the operation was conducted under the CIA's direction.