With the threat of additional terrorist actions looming as the United States and Britain bomb targets in Afghanistan, domestic security, already on high alert after the Sept. 11 attacks on the Pentagon and the World Trade Center, was tightened still more Monday. 

"Every American should be vigilant," said Attorney General John Ashcroft. 

In Washington, President Bush presided over the swearing-in of former Pennsylvania Gov. Tom Ridge as the new head of the Office of Homeland Security. 

"We will take strong actions aimed at preventing terrorist attacks and prepare to respond effectively if they do come again," Bush said. 

One lingering cause for alarm, suddenly revived with new evidence, was the case of a Florida man who died of anthrax poisoning over the weekend. Ashcroft said the case "could become a clear criminal investigation." 

Health officials originally thought Bob Stevens, 63, had caught the rare but extremely dangerous livestock disease by chance in the outdoors, but when anthrax spores turned up in Stevens' computer keyboard and in the nose of a co-worker at the offices of American Media, Inc., in Boca Raton, the FBI was called in and sealed off the building. Antibiotics to several hundred people as a precaution. 

There was another scare on Monday, when a traveler aboard an American Airlines flight from Los Angeles was tackled by co-pilots and other passengers after he tried to barge into the cockpit. The 30-year-old man was described as mentally ill, but two military jets escorted the plane to Chicago after the incident. 

At Las Vegas resorts, no chances were taken as guests were asked for permission to inspect trunks when handing over cars for valet parking. 

At Georgia's Lake Lanier, a security boat was on patrol to protect a major source of power and drinking water. In Arkansas, officials announced that the state fair would open Friday as scheduled — but backpacks would be forbidden. 

Terri Legg, 39, and Freddie Foster, 43, of Elmer, N.J., thought about driving to Philadelphia on Monday to shop and sightsee — but decided to shop at a mall closer to home. 

"If there's going to be an attack, they'll go for a place with thousands of people, and Philadelphia is very congested," Foster said. "We're looking to go to places where it's a little more safe, a little less crowded. We've told our kids to lock the doors, too. Before, it was like `Who would want to break into our house?' Now, you don't know." 

Jim Furlan and his wife, Cheryl, waited at Chicago's O'Hare Airport for the flight home to Los Angeles. Jim was OK with flying, if "a bit queasy." There was no way Cheryl would get on a plane — except that their two daughters, ages 3 and 5, awaited them. 

"I feel sick," she said. 

Estelle Faryon, 64, of Winnipeg, Manitoba, a visitor to San Diego, was on a cruise when she caught herself looking over the ship's rail and watching a fishing boat approach. 

"I was watching to make sure it didn't come too close. I never did that before," she said. 

The calls for gas masks and American flags picked up again at the Ax-Man surplus store in St. Paul, Minn. Although the store stresses the gas masks they sell are novelty items and not intended for legitimate use, manager Kim Mourning said the place has a waiting list. 

"Everyone from mothers to older people have called," she said. "Some don't even want to be on a list — they want it now." 

Around them, they saw signs that government was taking the threat very seriously. In downtown Los Angeles, newly placed concrete barricades manned by police officers protected City Hall. In Atlanta, police assigned to monitor public schools were put on heightened alert. 

Railroads restricted shipments of hazardous cargo, diverting them from heavily populated areas. Trucks and other vehicles were searched before entering downtown Minneapolis. 

"I don't want to stop my life. In the beginning, yes, I did things differently. Now I'm getting back to normal," said Gina Morgan, a 29-year-old accountant from Inkster, Mich., who visited Fairlane Mall in Dearborn to get her nails done. 

"I'm a little more aware of my surroundings. I look at people now, and I wonder, `Could they harm me or are they just shoppers or are they just casing the place?"' 

Nancy Fritz, a nursing student from Irvine, Calif., studied for her midterms in the courtyard of a Phoenix mall while her husband attended a convention. 

"For me, especially living near Hollywood, where most of America's culture is disseminated to the world, the possibility of retaliatory strikes now seems very real. I'll probably start to stockpile bottled water and nonperishable food supplies," she said. 

Still, "I can't see myself taking a vacation in Afghanistan any time soon, but other than that, I won't change much in my life." 

There is plenty of that kind of fatalism. "We're aware that anything can happen at any given time. But what can we do? I'm not scared. I continue to do what I have to do," said Crystal Drysdale of Temple City, Calif., as she rode the subway through Los Angeles with her friend, Sharon Cooper. 

"My mom has cancer right now," said Cooper. "I'm scared of that." 

But for others, carrying on was a patriotic and perhaps courageous gesture of defiance against accused terrorist mastermind Usama bin Laden. Claudette and Floyd Nelson, retirees from Mora, Minn., endured a 30-minute wait in bitter cold and passed through a metal detector to see the Liberty Bell in Philadelphia. 

"It's telling that little slug over there that we're not afraid of him," said Claudette Nelson, 67. "He's not going to succeed. No way."  

The Associated Press contributed to this report