The warnings have sent ripples of anxiety as far away as the West coast. Officials sealed off some streets in New York, put financial employees in Washington through extra security checks, and added concrete barricades and a heavily armed presence in Newark, N.J.
Yet The New York Times and Washington Post reported Tuesday that the information that led to the warnings may have been several years old, possibly from before Sept. 11.
"There is nothing right now that we're hearing that is new," one senior law enforcement official told The Post. "Why did we go to this level? ... I still don't know that."
"This information isn't new," White House Homeland Security adviser Fran Townsend told CBS News White House Correspondent Peter Maer. "We said when we put out the intelligence that casing reports were done originally between 2000 and 2001 but were updated as recently as January of this year. From what we know about how al Qaeda does business, particularly 9/11, they do their homework. They do it well in advance and they update it just before they're going to launch an attack."
Some of the more recent information may have come from publicly available sites on the Internet. But Townsend warned that reports about the age of the information should not convince people to let down their guard.
"The most dangerous thing about it is if it causes people to take the reports less seriously and not really pay attention to the security measures — that will be the greatest harm they do," she said, describing the evidence as "chilling" and "absolutely detailed."
Officials told the papers that, taken with general intelligence indicating that al Qaeda intends to strike in the U.S. this year, the three- or four-year-old information was sufficient to justify the massive security response in New York, Washington and Newark.
Police said the new security restrictions would remain in effect Tuesday and would be reviewed daily.
"You realize that's the world you live in, and you deal with it," said Kenneth Polcari, a trader at the New York Stock Exchange, one of many weary workers who filtered into the Wall Street building Monday.
The government triggered the concerns Sunday when it announced that terrorists had recently observed the stock exchange and The Citigroup Center in Manhattan, the International Monetary Fund and World Bank buildings in Washington, and Prudential Financial Inc.'s headquarters in Newark.
Since then, officials have acknowledged that the information came largely from a Pakistani computer engineer captured last month and that most of the information was amassed in 2000 and 2001. No timetable for potential attacks has ever been specified.
Mayor Michael Bloomberg and Gov. George Pataki rang the opening bell at the stock exchange Monday to show solidarity with workers and visited the Citigroup building with first lady Laura Bush and her twin daughters in the afternoon.
On Tuesday, Department of Homeland Security Secretary Tom Ridge along with Bloomberg, Pataki and New Jersey Gov. James E. McGreevey were to meet with financial executives from affected companies to discuss security concerns.
Plans toto the public went ahead Tuesday despite the new warnings; it had been shut since the 2001 terror attacks.
Greater security was more visible in the nation's capital, where authorities announced plans to block portions of Independence and Constitution avenues — major traffic arteries that run on either side of the Capitol grounds.
Washington police checked ID cards as employees filed into the World Bank headquarters; inside, security guards checked them again. Across the street, guards at the International Monetary Fund swept the underside of cars with detecting devices as they entered the garage.
"I'm concerned, but we have to carry on as normal," said IMF employee Shirley Davies.
Police closed Manhattan streets around Grand Central Terminal and banned trucks from some bridges and tunnels. Trucks passing landmarks or traveling on major thoroughfares also were subject to random searches.
Officials set up concrete barriers and police teams around the 24-story Prudential building in New Jersey, where about 1,000 employees work. They showed identification to get into the building and its underground parking garage.
Prudential chairman and CEO Arthur F. Ryan reported that customers are not fleeing and the "overwhelming majority" of employees reported to work. "Everything we've heard so far has been reinforcing. 'We're with you.' That's basically what we've heard from most of them," Ryan said.
In response to the East Coast terror alert, security measures were heightened in downtown Los Angeles and in the Century City complex where high-rise buildings and financial institutions are located.
"We have received no credible information to indicate any of these threats are directed against Southern California. However, we are going to continue to remain on a heightened state of vigilance," Los Angeles Mayor James K. Hahn said at a news conference.
The fears reverberated to the nation's heartland. The Chicago Board of Trade closed its visitors center in response to the threats, and the Federal Reserve Bank of Chicago also heightened security, said Federal Reserve spokesman Douglas Tillett.
Chicago Mercantile Exchange spokeswoman Anita Liskey also wouldn't comment on specific security measures at the futures market, one of the largest in the world, but employees in the building said they had noticed changes.
"There's more security guards," Mercantile clerk Jenna Fowler said, pointing to a guard standing outside a window. "It makes me scared that it's a financial institution, but there's a lot more security than there was last week."