In the 72 hours leading up to Sunday's warning about new risks of terror attacks, senior officials pored over a wealth of detailed new information indicating al Qaeda operatives were collecting chillingly detailed information about five financial-services buildings in the United States.
The trove of hundreds of photos, sketches and written documents came to light as a result of Pakistan's mid-July capture of Muslim extremist Mohammad Naeem Noor Khan, also known as Abu Talha.
Officials are now following investigative leads, as they try to learn more about possible plots against the apparent targets: The Citigroup Center building and the New York Stock Exchange in New York, the International Monetary Fund and World Bank buildings in Washington and Prudential Financial Inc.'s headquarters in Newark, N.J.
After analyzing the more than 500 pages of documents detailing al Qaeda's surveillance of U.S. financial targets, intelligence officers estimate truck bombs could have succeeded against the security measures in place before Sunday's alert, reports CBS News National Security Correspondent David Martin.
In other words, reports Martin, if al Qaeda could have constructed the bombs, it could have pulled off a second devastating attack on American soil.
On Monday, a Pakistani intelligence official said Khan, a computer and communications expert who refuses to say if he is part of al Qaeda, had sent messages to suspected al Qaeda members using code words — a practice typical of the international jihadist organization that bedeviled U.S. efforts to unravel the Sept. 11, 2001, plot.
Khan's information has been merged with other pieces of intelligence, including information gleaned after Pakistan's arrest last month of a senior al Qaeda operative named Ahmed Khalfan Ghailani.
E-mails that included plans for new attacks in Great Britain and the United States were found on the computer of the captured Ghailani, Pakistan's information minister, Sheikh Rashid Ahmed, said Monday.
But it was Khan and the information found after his arrest that was mostly behind the decision to raise the government's terror alert for financial-services buildings in New York, Washington and northern New Jersey — to orange, or high alert.
The FBI is analyzing the information about the surveillance of these five buildings, obtained after Khan's capture, to try to determine when it may have occurred, so that investigators can review building logs or videos during that same period, said one senior law enforcement official, speaking only on condition of anonymity because of the continuing investigation.
Investigators hope that logs or video might help identify some of the individuals involved, which would help agents understand the breadth of the terror plot.
The FBI is also trying to compare reports about the surveillance with previous intelligence reports that might have been considered innocuous or unconfirmed months ago, but might take on important new meaning in light of the latest information.
As authorities try to locate other people who may have been involved with the surveillance work and any subsequent plotting, one senior intelligence official said "a mosaic" of the plotters is slowly coming into focus. Still, officials have said the information that was uncovered did not reveal any specific cells or individuals in the United States now.
Important mysteries remain.
Officials, for instance, don't yet know precisely when the detailed surveillance work took place.
A counterterrorism official, speaking on the condition of anonymity, said authorities believe the surveillance was going on both before and after the Sept. 11, 2001, attacks, based on clues within the documents, including descriptions of security indicative of practices used before the suicide hijackings.
Homeland Security Secretary Tom Ridge said government analysis indicates al Qaeda would favor destroying the buildings, possibly using truck or car bombs. Such an attack is believed to be among the most difficult to defend against.
Ridge has a list of reasons theorizing why such an attack hasn't happened in the United States: Perhaps terrorists are focusing on large-scale, mass-death events or perhaps it's harder getting explosives and attackers willing to die in America.
Or, he said recently, "Maybe we are just lucky."
The documents that were seized reveal that al Qaeda was studying whether some explosive materials might not be hot enough to melt the steel underpinnings of a building. They discuss what materials might heat to 2,700 degrees, a senior intelligence official said Sunday.