Washington (CNN) -- The Obama administration has released more once-secret national security documents, this time detailing the origins of increased electronic surveillance to collect foreign intelligence in the months after the 9/11 attacks.
Director of National Intelligence James Clapper said Saturday in a statement that the material shows that President George W. Bush authorized spy agencies to collect contents of some overseas communications, as well as the bulk collection of domestic phone calls and e-mail metadata.
The 10 documents show deep concern within the U.S. government over the possibility that other terrorist cells were operating in the United States, plotting a domestic attack like the ones that struck New York, Washington and Pennsylvania on September 11, 2001.
One such document, from 2007, explains the government's concern: "We have numerous reasons to believe al-Qaeda is still plotting another attack on U.S. soil," then-DNI Michael McConnell says in a newly declassified declaration as part of an ongoing lawsuit over the telephone records. "Besides Bin Laden's own statements, his deputy, Ayman al-Zawahiri, issued threats against the United States and its interests and enjoined Muslims worldwide to take up violent jihad in more than a dozen audio or video taped statements in 2006 and at least four thus far in 2007."
McConnell adds, "The (U.S.) Intelligence Community assesses al Qaeda is most likely employing a diversified operational planning model, involving multiple, probably autonomous concurrent efforts."
The agency over the summer began selected release of documents, following a pledge by President Barack Obama to increase transparency.
Large sections of the newly released material were blacked out to "protect information that remains properly classified for national security reasons and because of the great harm to national security if disclosed," said the office of the Director of National Intelligence.
The disclosure that the National Security Agency has been collecting metadata has stirred provoked from civil liberty groups worried about the impact on privacy rights.
Bush's initial unilateral executive action in 2001 on the metadata collection was replaced by congressionally mandated updates to the Foreign Intelligence Surveillance Act. That revised law said federal judges working for a secret FISA court had to periodically approve the surveillance.
Officials have previously said the changes to intelligence gathering and data collection proved key to stopping further incidents of domestic terrorism.
"President Bush issued authorizations approximately every 30-60 days," the Director of National Intelligence office said Saturday in a statement. "Although the precise terms changed over time, each presidential authorization required the minimization of information collected concerning American citizens to the extent consistent with the effective accomplishment of the mission of detection and prevention of acts of terrorism within the United States."
Obama said Friday that he was considering making changes within the National Security Agency, after a panel appointed by him made specific recommendations.
Since former NSA contractor Edward Snowden leaked thousands of highly classified documents this year, the president and the intelligence community have promised greater public scrutiny of spying and anti-terrorist operations.