Key financial institutions in New York, Washington and Newark, N.J., were under heavy security Monday, one day after federal officials issued a warning that the buildings could be targets of an Al Qaeda terror attack.

"The elevation of the threat level in New York and New Jersey and Washington is a serious reminder, a solemn reminder, of the threat we continue to face," President Bush said at the White House Monday. "All the institutions of our government must be fully prepared for a struggle against terror that will last into the future."

Homeland Security Secretary Tom Ridge (search) told FOX News' "FOX & Friends" on Monday that intelligence officials had gotten unusually detailed information from multiple sources.

Ridge said it did not appear it would be a strike with a radiological or "dirty" bomb.

"Analysis suggests in this instance they prefer a car or truck bomb that they've employed elsewhere," Ridge said.

The warning stemmed in large part from Pakistan's capture of an Al Qaeda operative several weeks ago.

A cache of recently obtained information — including photos, drawings and written documents — indicates that Al Qaeda operatives had meticulously cased five specific buildings: The Citigroup Center headquarters 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.

Also, according to a law enforcement bulletin issued by the FBI and Department of Homeland Security on Sunday, there is concern that subways and other modes of public transportation near financial institutions could be targets for terrorists.

The bulletin, obtained by FOX News, also discusses the potential use of airplanes as weapons and about a possible terror plot to conduct computer attacks.

Officials encouraged people to continue their normal activities but to also remain vigilant. The stock exchange and the Washington institutions were open for business Monday.

Additional security measures were being put in place at the IMF and World Bank, as well as at the Bureau of Engraving and Printing and the Federal Reserve.

A stock exchange spokesman said increased measures since the Sept. 11 attacks have included barricades on all sides and checkpoints to enter the exchange building.

A spokesman for Citigroup, whose 59-story headquarters is among Manhattan's tallest buildings, 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.

A spokesman for Prudential said the company had increased its security before the threat.

"Prudential Financial is already coordinating with local state and federal law enforcement officials and has taken additional security precautions to ensure the safety of our employees, customers and visitors to our office locations," said company spokesman Jim Gorman.

Ridge said it would be impossible to shut down access to any potential target.

"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."

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.

New York City Police Commissioner Raymond Kelly said that starting on 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 lanes 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.

Washington Mayor Anthony Williams put the entire city on an orange alert, although DHS had not officially raised the threat level outside financial-sector buildings.

D.C. Police Chief Charles H. Ramsey said teams of bomb-sniffing dogs would sweep areas around the World Bank and IMF headquarters, and officers would conduct more traffic stops of large vehicles in the area.

Securing Manhattan

Kelly told FOX News Monday morning that based on that day’s reports, traffic in the areas most affected by the heightened security measures is "the same as usual."

"People are going to work, there's a lot of police presence throughout the city," he said. "Certainly down at the stock exchange where we've had it for quite a while, at Citigroup and a whole host of other locations throughout the city … you can talk to anybody who works down there [near the NYSE], they'll tell you that and now we've taken that model and moved it to other locations throughout the city."

What surprised everyone about the latest threat information, which is part of an "evolving stream" of information, Kelly said, was the specificity and detail of the possible threat.

For example, "some of this information talks about chairs in a conference room for instance," Kelly told FOX News, adding that people "clearly" have been scoping out the possible targets for long periods of time; others have said detailed plans could have started 10 years ago.

And "I think it's too early to say that these were the only sites that were uncovered in this trove," Kelly added.

Bloomberg and New York Gov. George Pataki spoke with reporters in New York Monday when they attended a groundbreaking for the new Bank of America building.

"Obviously there are very specific threats and very specific info that read to raising this terror alert," Pataki later told FOX News. Steps are being taken around those institutions mentioned and others "that would logically pose a potential target to those who want to take away our freedom."

"The security professionals are doing everything that can be done, based on the information we have, to protect the people in New York," Pataki continued.

One Homeland Security official told FOX News that the current threat information is so specific, it’s like "someone going through your home to work out how to attack you."

Other examples of information uncovered included the level of incline in target parking garages, traffic patterns, glass and metals that make up the buildings, location of elevators and the types of explosives that could be used to melt them.

"I don't like the colors [in the color-coded alert system] but I think we've gotten more specific, which is a good deal," said FOX News military analyst, Ret. Army Col. David Hunt, adding that even if homicide bombings are planned, the city is prepared.

"If that's the kind of guy you've got, you harden the target … you will not get near these buildings until this alert goes down," he said.

Terror Suspect’s Computer a Goldmine

An official said Al Qaeda's gathering of the building information took place both before and after Sept. 11, 2001.

FOX News confirmed 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.

Pakistani officials said details were on the computer of Ahmed Khalfan Ghailani (search), a Tanzanian arrested July 25 after a gun battle in the eastern city of Gujrat.

Authorities also arrested another top Al Qaeda suspect, a 25-year-old computer engineer, Muhammad Naeem Noor Khan (search), who apparently is cooperating with investigators.

Khan was arrested July 13. Pakistani officials say he used and helped operate a secret Al Qaeda communications system where information was transferred via coded messages, the New York Times reported Monday. FOX News confirmed with U.S. officials that Khan is the second man where information came from.

In fact, Khan is key to the latest security alerts, not so much Ghailani.

U.S. intelligence officials were expressing frustration that news outlets continue to point the finger at Ghailani, with one telling FOX News: "The assumptions are ridiculous. Just because Ghailani's name was the latest to cross everyone's radar screens ... it doesn't automatically mean he's responsible for all this. Even Ridge said 'no' yesterday when asked about the Ghailani connection."

A Pakistani intelligence official told The New York Times that Khan said couriers carried handwritten messages or computer disks from senior Al Qaeda leaders in isolated border areas to hard-line religious schools in Pakistan; others carried them to Khan himself to post them on Web sites or relay them electronically.

An anonymous senior intelligence official said the intelligence indicated Al Qaeda had evaluated security in and around the five 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.

The official said the Al Qaeda evaluations were so precise they included midweek pedestrian traffic counts of 14 people per minute on each side of the street for a total of 28 people. The official said he had not seen such extraordinary detail in his 24 years in intelligence work.

"Obviously, this is a war that's going to be won on intelligence," Rep. Peter Hoekstra, R-Mich., told FOX News on Monday. "We need our intelligence people to put these pieces together to make sure we can disrupt these activities to make sure there's not another attack."

FOX News’ Bret Baier, Catherine Herridge, Ian McCaleb, Liza Porteus, Anna Stolley and The Associated Press contributed to this report.