WASHINGTON -- FBI and CIA experts dug through piles of information Monday from the Pakistani home of alleged Sept. 11 mastermind Khalid Shaikh Mohammed, searching for clues that new strikes might be imminent.

U.S. authorities also questioned Mohammed on Monday, seeking information about safe houses and hideouts used by the al-Qaida terror network, a Pakistani intelligence official said. Mohammed's exact whereabouts were unclear.

Mohammed had been plotting attacks against targets in the United States and Saudi Arabia in the weeks before his capture, U.S. counterterrorism officials contended.

Such attacks might have been against commercial or other lightly defended civilian targets, officials said, although they acknowledged they do not know whether al-Qaida targets had been selected.

Intelligence about Mohammed's activities led in part to the orange alert that lasted most of February, Homeland Security Secretary Tom Ridge said.

"Some of the concerns we had that caused us to raise the threat level were attributable to the planning he was involved in," Ridge said. "There were multiple reasons that we raised the threat level and his relation to one of the plot lines was one of the several."

Ridge declined to discuss specifics but said the threat level was lowered last week because later information showed that plans for attacks had been disrupted and were less likely to occur.

Authorities recovered a huge amount of information about al-Qaida at the house in Pakistan where Mohammed and two others were arrested early Saturday, a senior law enforcement official said Monday.

Recovered at the home in Rawalpindi were computers, disks, cell phones and documents. Authorities believe the materials will provide names, locations and potential terrorist plots of al-Qaida cells in the United States and around the world.

Mohammed also was believed by U.S. officials to have details about the group's finances.

He was captured as he slept early Saturday. Pakistani Ahmed Abdul Qadus and an unidentified third man were also detained.

Officials expressed concern that al-Qaida cells could accelerate plots in the United States and elsewhere rather than run the risk of being captured. Or cell members might also go into hiding, believing their security was compromised by Mohammed's capture.

Mohammed, in his late 30s, is perhaps the most senior al-Qaida member after bin Laden and his deputy, Ayman al-Zawahri.

He is alleged to have organized the Sept. 11, 2001, terror attacks, and was linked to a 1995 plot to bomb trans-Pacific airliners and crash a plane into CIA headquarters and to the 1993 World Trade Center bombing. He also has been tied to the April bombing of a synagogue in Tunisia, which killed 19 people, mostly German tourists.