"What was very unique about this information … is the explicit nature, the detail, the thoroughness (and) the sophistication," Homeland Security Secretary Tom Ridge told CBS News Early Show co-anchor Julie Chen in an interview broadcast Monday. "That was what's very unusual."
It is impossible to shut down access to any potential target, Ridge acknowledged Monday. "We're the most open society in the world. People walk down our streets and through our neighborhoods," he said. "We don't always know who they are ... That is one of the great strengths of our country but it also is one of the great vulnerabilities."
A cache of recently obtained information — including photos, drawings and written documents — indicates that al Qaeda operatives have undertaken meticulous preparations to case five specific buildings: 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.
Following the U.S. warnings, the British government also said it was monitoring the "real and serious" threat from terrorism, but didn't warn of any specific risk.
Ridge on Sunday raised the terror threat level for financial institutions in the three cities to orange, or high alert, the second highest level on the government's five-point spectrum. Elsewhere, he said, the alert would remain at yellow, or elevated.
"Iconic economic targets are at the heart of (the terrorists') interest," Ridge said.
The fresh intelligence did not give crucial details about when, where or how terrorists may strike, Ridge said, but government analysis indicates terrorists may prefer to use car or truck bombs or other means to physically destroy targets.
Briefing reporters in New York, Police Commissioner Raymond Kelly said that starting Monday, trucks would be banned from the Manhattan-bound side of the Williamsburg Bridge, which connects Brooklyn and lower Manhattan. Commercial vehicles also were banned from the inbound of the Holland Tunnel from New Jersey, among other measures.
In Newark, police set up metal fences surrounding the Prudential Plaza building, blocked off two city streets and toted assault rifles.
And in Washington, Mayor Anthony Williams put the entire city on an orange alert, although the Homeland Security Department has not officially raised the threat level outside financial-sector buildings. Police Chief Charles H. Ramsey said teams of bomb-sniffing dogs would sweep areas around the World Bank and IMF headquarters, and officers will conduct more traffic stops of large vehicles in the area.
Officials have warned that the al Qaeda network, blamed for the Sept. 11, 2001, attacks, may launch a large-scale assault in hopes of disrupting the Nov. 2 elections and demonstrating that it remains capable of offensive actions despite international efforts to combat terrorism.
An example of that international cooperation, Sunday's warning stems in large part from Pakistan's capture of an al Qaeda operative several weeks ago, a U.S. counterterrorism official said, speaking only anonymously because of the sensitive nature of the information.
That detention led to the discovery of the documentary information — obtained during a successful CIA counterterrorism operation — about the extensive surveillance of the five buildings, the counterterrorism official said. Based on clues found within the information, the official said the scouting was going on before and after 9/11, but it's unclear how recently it took place.
Ridge would not comment on specific sources of the intelligence, but he credited strong partnerships with allies around the world, specifically citing Pakistan.
Pakistan's information minister, Sheikh Rashid Ahmed, told The Associated Press on Monday that Pakistani intelligence agents discovered plans for new attacks on the United States and Britain on a computer seized during the arrest of a high-ranking al Qaeda operative.
He said the plans were in e-mails on the computer of, a Tanzanian arrested July 25 after a gunbattle in the eastern city of Gujrat. The Pakistani official would not confirm whether the information prompted Sunday's terror warnings.
Ghailani was wanted for the 1998 twin U.S. embassy bombings in east Africa.
Authorities have also arrested another top al Qaeda suspect believed to be a computer and communications expert, and that that man was cooperating with investigators, Ahmed said.
A senior intelligence official, also speaking on condition of anonymity, said the intelligence gathered from several sources indicated scouting had been done to identify security in and around these buildings; the best places for reconnaissance; how to make contact with employees who work in the buildings; traffic patterns; and locations of hospitals and police departments.
As one senior intelligence official put it, "every vulnerability has been identified."
The official tells CBS News Correspondent Bob Orr that the information reveals terrorists have studied even the smallest security details. One piece of information concerning one building notes, "there are three security personnel located upstairs. Only one carries a weapon."
Officials in New York, New Jersey and Washington encouraged people to continue their normal activities but remain vigilant. The stock exchange and the Washington institutions were to open for business Monday, as was Prudential, with employees expected to report to work.
Treasury Secretary John Snow said the U.S. financial system continues to operate normally, adding that there are sufficient safety mechanisms to ensure that any terrorist attack on the U.S. financial system can be weathered.
A spokesman for Citigroup, whose 59-story headquarters is among Manhattan's tallest, cited a company e-mail to employees that said they should expect to see tighter security at all of the company's New York buildings.
As CBS News Correspondent Randall Pinkston reports, the warning and New York's tragic history have security experts like Robert Strang in a quandary: "How do you tell someone to go into an office building that's been targeted by the same group that killed 3,000 people three years ago?"