In ways big and small, obvious and opaque, already tight security has been fortified at transportation and energy facilities nationwide in response to government warnings that al-Qaida could attack this holiday season.
"Terrorists still threaten our country and we remain engaged in a dangerous - to be sure - difficult war and that it will not be over soon," Defense Secretary Donald H. Rumsfeld said Tuesday.
He said raising the terror alert costs money and causes stress on military and civilians at all levels of government. "You do not do it lightly," Rumsfeld said. But he also echoed remarks by other Bush administration officials encouraging Americans not to change their plans.
"We're free people," he said. "We shouldn't sit around hiding under chairs. ... We have to live our lives and that's what we'll all do."
Across the country, stepped-up security was in place at airports and other sites that could be targeted for terrorist attack.
Bomb-sniffing canine units were added at Cincinnati/Northern Kentucky International Airport, maritime patrols were augmented near power plants that border Lake Erie, and more officers than usual were activated along the U.S.-Canadian border.
U.S. officials said Tuesday they have received information from a credible source about an al-Qaida threat against oil interests in Alaska, which they have not fully corroborated. Still, officials were treating the information seriously and have taken extra security precautions.
A Coast Guard spokesman, Roger Wetherell, said air and water patrols have been stepped up around the Port of Valdez, where tankers depart carrying Prudhoe Bay oil to the lower 48 states, and that vessel boardings will increase.
While he declined to detail security measures taken, Gen. Richard Myers, the chairman of the U.S. Joint Chiefs of Staff, said Americans could see "additional air patrols over select cities, and increase in the air defense posture here in Washington, D.C. and combat aircraft could be put on a higher alert at different air bases throughout the country."
Similar defenses have been increased around key bridges, tunnels, seaports and landmarks, as well as chemical facilities and other places that may be vulnerable to attack. Other layers of protection likely have been in place since Sunday, when the national security level was bumped up to orange, which is "high" alert, from yellow, or "elevated," security experts said.
"There's going to be a menu of visible and invisible measures that are implemented," said Brian Jenkins, research associate at the Mineta Transportation Institute at San Jose State University and a special adviser to the Rand Corp., a California-based think tank.
Among the covert steps likely taken, Jenkins said, are an increase in the number of air marshals, particularly on flights arriving from overseas, undercover surveillance around airports and more frequent air patrols near major cities.
There are some concerns about missiles being fired at planes taking off or landing, Jenkins said.
President Bush said the government was doing its best to protect the country and advised citizens "to go about their lives."
"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 in a statement.
The fact that it's the holiday season partly drove the decision to raise the alert. During this time of year, many people are distracted and traveling, and several gather at events like football games and New Year's celebrations.
However, authorities also said the move was based on specific, corroborated intelligence that al-Qaida may soon try to pull off an attack in multiple places to cause mass casualties.
White House spokesman Scott McClellan said such an attack could eclipse that of Sept. 11, 2001, which left nearly 3,000 people dead.
U.S. intelligence and law enforcement officials, speaking on condition of anonymity, said that they did not have any specifics about a potential method, location or time of any attack. They noted that in light of the Sept. 11 attacks, aviation could be a prime possibility.
Defense Department officials said they were launching more military air patrols over major cities, but they would provide no details. Other government sources said, however, that New York, Washington and Los Angeles were likely choices.
The Transportation Security Administration, the federal agency charged with protecting air travelers, would not discuss its planning at a detailed level, other than to confirm that vehicle inspections and parking restrictions have increased at airports and that Coast Guard patrols have been added near airports bordering oceans and lakes.
The Nuclear Energy Institute, which represents the owners of more than 100 reactors in 31 states, said the power industry is coordinating with law enforcement and intelligence agencies and has 7,000 of its own patrolmen at the ready.
In addition, cargo planes and flights originating overseas were of special concern in the latest warning.
To defend against another airborne attack, baggage and passenger screeners are working overtime at airports, and their managers have come out from behind the scenes to work the front lines, the TSA said.
The TSA also reminded travelers that it needs help, broadcasting messages over loudspeakers at airports and train stations that urge Americans to report any suspicious activity or unattended luggage.
The Coast Guard, meanwhile, cautioned boaters to avoid off-limits areas near power plants and to keep their eyes peeled for suspicious activity.
Amtrak said police officers are randomly riding and inspecting trains and that patrols have been beefed up inside stations and along platforms. Some baggage is also being checked, a spokesman said.