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, N.J.
The New York Times and Washington Post reported Tuesday that the information that led to the warnings may date to 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."
But top Bush administration officials said some of the surveillance was apparently updated as recently as January of this year.
"We know that this is a terrorist organization that does its homework," Homeland Security Secretary Tom Ridge said in a news conference Tuesday morning. "Al Qaeda often plans well in advance. We also know that they like to update their information before a potential attack."
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.
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.
But White House Homeland Security adviser Fran Townsend warned CBS News Correspondent Peter Maer 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 said that, taken with general intelligence ndicating 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.
"We felt it was essential to share it with the appropriate communities and the companies so they could act upon it," Ridge said on Tuesday.
"The detailed, sophistication, thoroughness [of the information], if you had access you'd say we've done the right thing," he added.
Ridge answered critics who suggested the timing of the alert, right after the Democratic convention, was political motivated.
"We don't do politics in the Department of Homeland Security," he said.
"Frankly, I would point out that this is the most significant, detailed pieces of information about any particular region that we've come across in a long, long time. That's why we needed to share this publicly," Ridge added.
In the three cities named in the evidence, 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.
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.
However, plans toto the public went ahead Tuesday despite the new warnings; it had been shut since the 2001 terror attacks.
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.
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.
Ridge said intelligence and security officials realize there are al Qaeda operatives and sympathizers within the United States.
"We have to accept for our planning that they're here, looking to attack us, and we need to do everything we can every single day to detect, deter and prepare for it," he said.