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, reports CNN.
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.
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, informs CNN.
US authorities have blocked many people from entering the country from abroad without any reasons.
This time, officials claim that the threat is way more serious than previous alerts. 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.
President Bush however appears calm and says that “Americans need to go about their lives as usual,” reports CNN.
The airport is expected to handle 2.6 million passengers between this past Friday and January 6.
"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?"
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.