The Sept. 11 attacks shattered the American sense of freedom, casting the very idea of freedom in a new light.
They also raised the question of whether Americans should be willing to surrender some of their rights in exchange for security against terror.
Some lawmakers and security experts say, yes, absolutely -- despite a drumbeat from privacy advocates who say it doesn't have to be that way.
"With regard to civil liberties, what people refer to as civil liberties, I think there are certain things that need to be given up to a certain extent," according to radio talk show host Michael Smerconish.
Americans may take for granted that they are protected from government intrusion. But federal agents have enjoyed expanded investigatory powers in the wake of Sept. 11 that allow them more than ever to track e-mail, poke through financial transactions, peruse library and consumer histories -- even sign up neighbors to spy.
"I believe some of these tools would have helped in the events of 9/11," said security consultant Bill Daly. "I'm not saying it would have stopped 9/11 from occurring, but they would have been helpful tools for law enforcement and intelligence agencies. I think now the time is right to be able to use them."
Before Sept. 11, the federal government already had wide-ranging powers to secure warrants. But new authority under the USA Patriot Act has broadened those powers, and has made it easier for agents to conduct their surveillance.
Critics say law-abiding citizens' rights are being abridged, while terrorists will continue to subvert the law. They also fear the government is setting a dangerous precedent with new spying powers that might be abused in the future by less well-meaning administrations.
"I think it raises real serious concerns for the potential precedents they set and how they might be abused down the road," said James Lyndsey, an analyst with the Brookings Institution.
"I think if you actually look at Sept. 11 ... it's not at all clear that civil liberties were the problem," Lyndsey continued. "What really seemed to be the problem was foremost a lack of vigilance, a real lack of conviction within the American public, within the federal government, that a catastrophic terrorist attack could really happen here in the United States."
But that's why Americans should not be wary about giving the government the tools it needs to prevent another terrorist attack, say supporters of increased security measures.
"This country needs a gut check," said Smerconish. "Are we at war or are we not at war?"
The Patriot Act also allows the government to indefinitely detain immigrants -- even permanent residents -- without criminal charges. Profiling for Middle Easterners and Arabs has become a major debate as airport officials and law enforcement officials try to plug the holes in the nation's security.
Some point out the terrorists who committed the heinous Sept. 11 attacks share a common thread: They are all radical Muslims with Middle Eastern and Arab backgrounds. So, they reason, targeting men who fit that description is not only common sense -- it's a necessary law enforcement tool.
"Usama bin Laden has called on all Muslims to destroy Americans," said Heather MacDonald, of the Manhattan Institute. "He has not called on all Jews or Catholics to destroy Americans -- it's a Muslim appeal. Law enforcement needs to keep that in mind."
Many Americans seem to agree. A Fox News/Opinion Dynamics Poll found that 54 percent of Americans favor profiling Arabs or Muslims outright. Only 34 percent thought it was a bad idea.
Arab-American advocates have a different take on the situation.
"A profile is a whole bunch of behavioral characteristics of which race or religion or gender may be one," said Jean Abinader, of the Arab American Institute. "The problem is when you give it to people who are untrained in the situation or who are looking for shortcuts, race becomes the dominant feature and that's where profiling fails."
John Whitehead of the Rutherford Institute says treating potentially innocent people as though they were criminals is not the American way.
"What we're saying is we're willing to subject innocent people to suspicion. Treat them as if they were criminals," he said. "The question is, where do we stop? It completely guts our Constitution's freedoms."