In the past year, jets have been transformed into missiles while a bacterium has been used as a poison and shipped through the mail.
While much has been done to guard against another attack since Sept. 11, security experts say one certainty is that terrorists will try again.
The question on many Americans' minds is: How safe are we? For any individual American, experts say, the answer is you face little personal threat. But the nation, though safer than it was a year ago, remains extremely vulnerable.
Just list the ways an attack could occur: a lone gunman with automatic weapons, a trio of suicide bombers in a crowded airport, a chemical attack spread in a mall's ventilation system, a dirty bomb of leftover radiological material slipped onto a commercial cargo ship, a group of suicide bioterrorists who sicken themselves with smallpox and wander through several cities, a stolen nuclear warhead detonated at a downtown dock.
Keep risk in perspective
"If Sept. 11 demonstrated anything, it's that it is illusory that we can wrap ourselves in a security blanket," said Bruce Hoffman, a counterterrorism expert at the nonprofit Rand Corp., and a consultant to the federal government.
"Terrorism, and particularly that of al-Qaida, is the archetypal shark in the water that has to keep moving forward to stay alive."
Yet experts note that the danger to any one person must be kept in perspective -- auto accidents killed 41,821 people in 2000 while last year's anthrax attacks killed five.
"You accept certain risks," said Jerome Hauer, who oversaw emergency management in New York and now heads the Public Health Preparedness Office for the federal Health and Human Services Department.
"There's a similar risk when you get in the car in the morning. You assume that the tractor trailer driver will drive safely, that the drunk driver won't hit you head on."
And there are ways to cut the odds.
U.S. Customs officers are headed overseas to check cargo containers as they're loaded in Singapore; local police and FBI are sharing more information about threats and suspects; hospitals and public health departments are tracking diseases for potential bioterror attacks; new laws require traders and bankers to more thoroughly track the people behind the money.
The changes aren't only in big cities. In Utah's rural Iron County, with a small airport in the desert, a new mindset has taken root.
"Now, I recognize that once you're through security in a little tiny airport, you have access to any airport in the world," said Sheriff Dude Benson, whose county has 35,000 people scattered over 3,300 square miles.
From police on the streets to those who train emergency responders, there's greater attention to little details that could be a tipoff.
A new mindset
"It's something my officers think about every day," said Detective Gary McLhinney in Baltimore. "That car stop with the tag light being out. Take it a step farther and see who's in that car, make sure the license checks out.
"They understand that if something's going to be averted, it's going to be because a street cop is going to stumble on it."
Before Sept. 11, teaching medical professionals and emergency responders about the threats of chemical and biological warfare, Dr. David Franz said he often saw two responses -- apathy or panic.
Now, "I see common-sense questions and concerns, and 'How can we deal with this?' and 'We're going to do it.' Education has done a lot already," said Franz, a professor, a former Army colonel at Fort Detrick and a weapons inspector in Iraq and Russia.
Finding the balance
The awareness of a threat isn't just on the front lines. A poll conducted for The Associated Press by ICR/International Communications Research of Media, Pa., found 63 percent of people felt another terrorist attack was very or somewhat likely.
Betty Schuster, a retired high school history teacher in Waterford, Mich., doesn't think all the police and FBI agents and customs guards can ultimately stop a terrorist from killing more Americans. She's angry and a bit scared, but she doesn't lose sleep or stop flying. She's found a balance.
"I really do feel safe," she said.
She stays alert when she travels, while out shopping, in her neighborhood. But she worries -- just like the experts working on this every day -- about the nation letting its guard down.
Said Hauer: "We forget that groups like al-Qaida are very patient. The sad thing is, I think we will have another event. And that will stem the tide of complacency."