For thousands of airline passengers having to toss out bottles of drinking water and cans of hair spray, there were delays and fears and frustrations today. And for security officials, another question loomed: One plot to blow up planes may or may not have been foiled -- but what other threats may loom out there?
"Our society was not built, our infrastructure was not built with terrorism in mind," said Michael Taylor, head of American International Security in Boston.
Which revives questions that came up after the terrorist attacks of 9/11: If planes are vulnerable, what about trucks? Or bridges? Or shopping malls? Or the food supply?
In many cases, private security consultants and other experts say it is hard to kill large numbers of people in one act. But with Americans already on edge, it may not be hard to do something that would scare millions.
"That's the fear factor," said William Vorlicek, associate managing director of Kroll Inc., a New York security consulting firm. "The terrorists have succeeded because people are scared."
Government officials, including Tommy Thompson, the former Secretary of Health and Human Services, have worried in particular about attacks on the food supply.
Thompson insisted America's food is safe. But in comments he made on Dec. 7, 2004, shortly before stepping down from office, he said, "That doesn't mean somebody, somewhere or sometime could not put some kind of adulterated additives into the food that could cause problems."
Security experts have described scenarios in which someone might dump a small amount of bacteria into one tanker truck, carrying a load of milk from a farm to a pasteurizing plant. Thousands of people might become ill. Few of them would die -- but people across the country would worry about feeding milk to their children.
Similarly, say consultants, it would be hard to poison a city's water supply. They also point to the extremist group in Japan that released sarin gas into the Tokyo subway system. Twelve people died -- but later investigation revealed that the group had hoped the gas would spread throughout the city. Instead, it never got beyond the one subway station where it was released.
Vorlicek, a former Army colonel who advised the New York state and city governments on how to protect against weapons of mass destruction in the 1990s, said terror groups were concentrating on airliners because they believed it would not take much to bring one down.
"There are some critical junctures in the plane that cause the hydraulics, the flaps and so forth to work. It would only need to be a small amount" of explosive, he said.
"What's scary is, this is stuff that had been talked about many years ago, before the first World Trade Center bombing, and now it's resurrected again. The thing to be remembered is, these guys are looking to succeed," Vorlicek said.
The best advice, Vorlicek said, is for people to keep an eye out for suspicious behavior, even while going about their regular business.
"There's a fine line between vigilant and paranoid," he said. "There's not a boogeyman behind every bush -- but what do you do at a stoplight? You look both ways. You've got to be vigilant."
"We live in a dangerous world," President Bush said during a trip to Green Bay, Wis., today. "This country is safer than it was prior to 9/11. We've taken a lot of measures to protect the American people, but obviously, we're not completely safe because there are people that still plot and people who want to harm us for what we believe in."