Alleged Al Qaeda chieftain Khalid Shaikh Mohammed was actively planning new attacks against commercial targets in the United States and the Arabian peninsula when he was captured over the weekend, U.S. officials said Monday.
Intelligence about Mohammed's activities led in part to the orange alert that was in effect for most of last month, the officials added.
"Some of the concerns we had that caused us to raise the threat level were attributable to the planning he was involved in," Homeland Security Secretary Tom Ridge said Monday. "There were multiple reasons that we raised the threat level and his relation to one of the plot lines was one of the several."
A U.S. intelligence memo dated Feb. 26 warned Mohammed was overseeing plans to attack suspension bridges, gas stations and power plants in New York and other major cities, Newsweek reported Sunday.
Authorities recovered a huge amount of information about Al Qaeda at the house in Pakistan where Mohammed and two others were arrested early Saturday, a senior law enforcement official said Monday.
Included in the haul were computers, disks, cell phones and documents that authorities believe will provide names, locations and potential terrorist plots of Al Qaeda cells around the world. Mohammed is also believed to have details about the group's finances.
FBI and CIA officials were working around the clock to pore over the seized information to prevent any imminent attacks and to determine when and if more arrests could be made.
Officials feared that even as they work feverishly on the case, some of the Al Qaeda operatives might go into hiding or halt any terrorist plots. Mohammed's arrest could also have the opposite effect of pushing Al Qaeda cells to accelerate plots in the United States and elsewhere rather than run the risk of being captured.
Experts said Mohammed's information about impending terrorist operations and the location of Al Qaeda leaders and cells grows more dated by the hour.
Whether the CIA can learn anything useful from the alleged Sept. 11 mastermind depends on the skills and methods of the interrogators, Mohammed's willingness to talk and perhaps simply time.
The interrogators' top priority is to gain intelligence that could disrupt attacks being planned or lead to added precautions.
That could mean a domestic law enforcement raid to break up a cell ready to strike, or an increase of security at areas Mohammed names as targets subject to imminent attacks.
Overseas, it could mean an operation that leads to the capture of Usama bin Laden.
But such information is just what Mohammed is most likely to try to keep secret, or lie about. Still, terrorists who learn of Mohammed's capture may alter their plans, abandon safe houses or make hurried telephone calls -- actions that could expose them to detection.
The only Al Qaeda capture that approaches the magnitude of Mohammed's was that of Abu Zubaydah last March. Zubaydah more than once provided information that sent American security officials scurrying to provide warnings to cities and sectors of the economy, knowing all the while that he could be lying.
Officials were not releasing details of Mohammed's detention. Previous high-level Al Qaeda captives have not been brought to U.S. soil, where they would have rights not afforded on foreign soil. Where they are, however, has not been disclosed.
Another secret is what means officials will employ to try to get Mohammed to talk.
Officials believe he can detail how Sept. 11 was put together, answering long-standing questions about the plot's origins: Who chose the World Trade Center and Pentagon as targets? Who picked Sept. 11 as the date?
American officials say Mohammed, who was born in Kuwait and has both Pakistani citizenship and ancestry, planned and coordinated key aspects of the Sept. 11 operation.
What he says can be cross-checked with information provided by Ramzi Binalshibh, his former aide who was captured in September. Binalshibh was a part of the cell that included Sept. 11 hijacker Mohamed Atta.
In the mid-1990s, Mohammed also worked with his nephew Ramzi Yousef and two others in the Philippines on a number of operations. Yousef is in prison for his role in the 1993 World Trade Center bombing.
One plan called for blowing up a dozen trans-Pacific airliners in flight. A second involved crashing an airplane into CIA headquarters outside of Washington. Officials have suggested these plots -- broken up in their infancy with the arrests of Mohammed's associates -- were the seeds of Sept. 11.
The four plotters were linked to Al Qaeda through a financial operative named Khalifa, who is bin Laden's brother-in-law, officials have said. Khalifa is believed to remain at large.
The Associated Press contributed to this report.