For the NSA, Bush's executive order authorizing the interception of electronic communication without warrants, signed in late 2001, represents a dramatic shift from restrictions on domestic spying imposed after exposure in the mid-1970s of NSA operations against U.S. citizens.

Before Bush's secret order, the NSA operated under strict limits on domestic intelligence collection. The Foreign Intelligence Surveillance Act (FISA) of 1978 set up a secret federal court that must approve requests for the NSA to conduct surveillance against anyone in the USA suspected of being an "agent of a foreign power," such as a terrorist group.

Revelations in 1975 of CIA misdeeds led to an investigation by a committee headed by then-senator Frank Church. The committee published a report in 1976 that uncovered three cases of NSA spying on Americans.

"Here we are, 30 years later, revisiting the whole issue," said Matthew Aid, a historian who has written about the NSA.