WASHINGTON (CNN) -- Surface-to-air missiles were being deployed around Washington, state troopers were authorized to ride New York area commuter trains, and air travelers encountered more delays -- all results of the heightened terrorism threat alert level.
On Sunday, the Department of Homeland Security raised the level from elevated, or yellow, to high, or orange, the second-highest level on the department's five-color warning system. (Full story)
The move was based mostly on information gleaned from a high volume of "chatter" among suspected terrorists, Homeland Security Secretary Tom Ridge said.
Ridge warned of possible strikes more devastating than the al Qaeda airliner attacks of September 11, 2001, on New York and Washington, and told CNN on Monday that airplanes remain terrorists' weapon of choice.
"There is a continuous stream of reporting, literally from the last two years, that indicates [terrorists'] preference or desire to use aircraft as a means of attack," Ridge said on CNN's "American Morning."
In addition to Washington, anti-aircraft missile batteries may be deployed around New York City, and a senior Pentagon official said "irregular air patrols" had been ordered.
Governors have been given lists of specific measures and sites to be protected, including critical infrastructure that if hit by an aircraft could cause extensive problems, an official said.
A government official said numerous people have been blocked from entering the country at various locations since December 1 but would not give specifics.
Another U.S. official said a handful of foreign flight crew members had been stopped in recent days.
The United States is working with airlines and foreign governments to improve their security postures, officials said.
There is concern about Mexico and Canada because of their contiguous airspace with the United States.
More serious this time
Officials at all levels said the threat this time around seems more serious than previous alerts.
One senior Pentagon official described the terrorism threat level as "true orange," noting that some previous orange designations had been viewed with skepticism.
There are particular concerns about Washington, New York, Los Angeles, Las Vegas, Nevada, a handful of other cities and two rural areas -- one in the East and one in the Southwest, officials said.
Some homeland security officials predicted the increased security could last into February, at a cost of about $1 billion per week -- much of it in increased overtime costs for law enforcement personnel.
On Tuesday, the Pentagon will conduct a continuity-of-government exercise as part of the orange alert, officials said.
Key officials will be told to move to secret alternative facilities. It was not clear how many people would be involved or whether they would move by helicopter or motor vehicle.
A senior Defense Department official described the intelligence that led to the orange alert as "specific, reliable, credible."
"From many, many sources from around the world, we've seen a significant increase in the level [of chatter] and the number of reports with regard to imminent threats against the homeland," Ridge said.
"Multiple sources [are] basically saying the same thing -- talking about near-term attacks the scope of which would be similar to or even greater than September 11."
Government sources said some of the intelligence came from Britain.
The United States last raised the domestic terrorism threat level to orange May 20, after suicide bombings in Saudi Arabia and Morocco blamed on al Qaeda. That alert lasted 10 days before the threat level was returned to yellow.
Other orange alerts were raised in 2002 around the first anniversary of the September 11 attacks and in February 2003, on the eve of the U.S.-led invasion of Iraq in March.
President Bush said Americans needed to go about their lives as usual.
"But as they do so, they need to know that governments at all levels are working as hard as we possibly can to protect the American citizens," Bush said Monday at a menorah-lighting ceremony in the White House. (Full story)
New York Gov. George Pataki said he signed an executive order to allow state troopers from New York, Connecticut and New Jersey to ride commuter trains.
In addition, he told reporters, "we've increased patrols along the Canadian borders and ... are working to make sure everything from airports to bridges, tunnels and other mass transit facilities have heightened security."
New York Gov. George Pataki and Mayor Michael Bloomberg shop in Times Square on Monday to show confidence in security.
But Pataki urged visitors and residents to get out and enjoy the season. "Go to the restaurants, see the shows; there's no more exciting or better place to be."
Los Angeles Mayor Jim Hahn said security was heightened at Los Angeles International Airport and the Port of Los Angeles.
Some travelers were annoyed because of delays, but many took the changes in stride. (CNN/Money: Alert hits airlines)
"I travel international, and I don't feel it's giving me a lot of comfort," said one Los Angeles flight passenger. "It seems like there's a lot of confusion."
The airport is expected to handle 2.6 million passengers between this past Friday and January 6.
One woman at Miami International Airport said she was resigned to coping.
"The line seems to be pretty long, but it doesn't seem to be bothering too many people," she said.
"We have to travel, we can't live in fear, and you know they raise this level to orange somewhat with regularity. But what really can we do about it, other than just travel?"
In Miami harbor, submarines and dive teams are inspecting the undersides of ships. Four million passengers and 9 million tons of cargo come through the port each year.
The heightened security concerns involved U.S. interests overseas as well.
U.S. military bases have raised their force protection levels from "Alpha" to "Bravo," a Pentagon official said.
New York City police officers stand guard at the New York Stock Exchange early Monday.
Specific bases were not named. The change means more perimeter patrols and security checks will be performed on people and vehicles entering and leaving the facilities.
U.S. military intelligence officials said al Qaeda terrorists may be in the final stages of planning an attack in Saudi Arabia.
Last week, the State Department authorized nonessential diplomats and families of U.S. officials to leave Saudi Arabia because of ongoing security concerns, and urged Americans to defer travel to the kingdom.
Terrorism expert Brian Jenkins of the Rand Corp. told CNN that the terrorists' goal -- whether at home or at U.S. interests abroad -- is to affect every American.
"The target of it is every one of us -- our continued commitment to values, for which this country stands," he said.