In the years since, a host of new security measures have been put in place to stop other attacks. Most have become so part of everyday life that we don't even notice them anymore. But some raise fundamental legal and constitutional questions with no easy answers.
That was the case in November, just before Thanksgiving, when new security procedures went into effect at some airports: Passengers had to walk through full-body electronic scanners or submit to extensive pat-downs, and many people objected.
Striking a balance between protecting the public from very real terrorist threats and safeguarding civil liberties has been the subject of intense debate since 2001. It has played out not only at airports but in many other areas of American life, with Congress, the courts, and two Presidents trying to figure it all out.
In 2002, for example, President George W. Bush authorized the National Security Agency to monitor the phone calls and e-mails of hundreds, perhaps thousands, of Americans and others inside the U.S suspected of terrorist ties, without first obtaining warrants.
When the program became public knowledge in 2005, a political firestorm erupted. Opponents cited the Fourth Amendment's protections against "unreasonable searches and seizures," which has generally been interpreted to mean that government authorities must obtain court-issued warrants before conducting wiretapping or other such monitoring.
Bush claimed that the program was a legitimate exercise of presidential power, including his constitutional authority as Commander in Chief. Security officials also cited concerns about situations in which delaying monitoring for even a few hours while waiting for a warrant could result in the loss of critical intelligence that could stop an attack.
Last March, however, a federal judge ruled that warrantless wiretapping is illegal. The Justice Department says the practice has been discontinued under the Obama administration.
Questions have also been raised about the hundreds of terrorism suspects who have been held since 9/11 at the U.S. Naval base in Guantánamo Bay, Cuba, and whether they are entitled to the same legal protections as Americans.
Some of the detainees at Guantánamo are undoubtedly dangerous and present real threats to the U.S. But others may have just been in the wrong place at the wrong time. It's hard to know, since many have been held for years without triala violation, civil liberties groups say, of the Fifth Amendment guarantee of "due process." The government has classified the detainees as "enemy combatants" and says they are not entitled to constitutional protections because they're not on American soil.
When President Obama took office in 2009, he promised to close Guantánamo within a year and respect American values in the fight against terrorism. "We intend to win this fight," he said. "We are going to win it on our own terms."
But Obama seems to be having just as hard a time figuring out what to do with the detainees as his predecessor. While the Supreme Court ruled in 2008 that Guantánamo detainees have the right to challenge their detention in federal courts, many questions remain, including figuring out how, where, and if to try the detainees.
As the courts and lawmakers deal with Guantánamo, they are wrestling with two big questions: Do U.S. constitutional protections apply to terrorism suspects? And to what extent are we willing to violate a suspect's rights, including the use of harsh interrogation methods, in order to keep a potentially dangerous terrorist behind bars?
But while wiretapping and Guantánamo are abstract issues for most Americans, what it takes to get on a plane affects the public in more direct, concrete ways.
"If we increase the use of wiretapping, that's quite invisible to people, so it doesn't tend to produce the same kind of outcry as when it takes a very long time in order to get sexually assaulted to get onto an airplane," says Benjamin Wittes of the Brookings Institution, a think tank in Washington, D.C.
About 400 full-body scanners are now in use at 70 U.S. airports; that number is expected to grow to 1,000 by the end of the year. Scanner opponents say the machines capture detailed images of naked bodies that are examined by strangers and can be electronically stored; some also cite health concerns about radiation. "It's very invasive, and the thought of going through that every time I fly is discouraging," says Laura Seay of Atlanta, who travels frequently for work.
But George Oberle, a Lutheran minister, says he gladly underwent the full-body scan when he recently traveled from Detroit to New York. "If it's going to keep me and others safe," he says, "I'm all for it."
Fourth Amendment Questions
Legal experts are just as divided as travellers about the new security measures. The Electronic Privacy Information Center has filed suit against the Department of Homeland Security, arguing that the body scanners violate Fourth Amendment protections against "unreasonable searches and seizures" as well as other federal laws.
"For Fourth Amendment purposes, you can't touch somebody like this unless you're checking them into a jail or you've got reasonable suspicion that they've got a gun," says John Wesley Hall, a criminal defense lawyer who specializes in search and seizure issues. "Here there is no reasonable suspicion; it's the pure act of getting on a plane."
But Orin S. Kerr, a law professor at George Washington University in Washington, D.C., says the courts have generally supported the government's claims on security needs in cases involving airport screening.
The problem, explains Benjamin Wittes at Brookings, is that security measures "tend to get more extreme over time, because every time you introduce a new one, the bad guys start working on a way around it."
And then the struggle to find a new balance between keeping us safe and protecting our liberties begins anew.
(The New York Times Upfront, Vol. 143, January 31, 2011)