That question has yet to be fully answered.
Investigators last week were chasing reports of possible car bombings planned for New York and Washington to mark the anniversary. Initially, the Obama administration had said it had no evidence any terrorist attack was imminent despite papers captured in the raid on Osama bin Laden's compound in Pakistan that indicated the al-Qaida leader wanted to make a splash around Sept. 11.
On Sept. 11, 2001, 19 hijackers took over four United and American airlines flights from Boston and New York. Two of the planes crashed into the upper floors of the twin towers of the World Trade Center, killing nearly 2,800 people. One plane crashed into the Pentagon, killing 125 people. Aboard the final plane, passengers revolted as the hijackers attempted to head the plane for either the White House or Capitol. The jet went down near Shanksville, Pa., killing all aboard the plane.
"The perpetrators of those attacks wanted to terrorize us, but they are no match for our resilience. Today, our country is more secure and our enemies are weaker," President Obama said in a recent op-ed piece. "Yet while we have delivered justice to Osama bin Laden (he was killed in the May 2 raid in his compound) and put al-Qaida on the path to defeat, we must never waver in the task of protecting our nation."
Numerous memorials were planned for Sunday.
A Harris poll of 2,073 adults released last week indicates nearly a third of Americans are more concerned about their personal safety now than they were 10 years ago and 84 percent said they expect the United States to be hit by a terrorist attack from a foreign citizen or organization in the next 10 years. Perhaps even scarier, 79 percent said they expect a terrorist attack will come from a U.S. citizen, a la Timothy McVeigh's deadly 1995 truck bombing of the Oklahoma City federal building.
A pair of Gallup polls last year indicated relatively few U.S. air travelers (27 percent) are upset about security procedures, but 71 percent said they'd prefer it if security procedures included profiling rather than depending on random selections for more intense security screenings.
University of Illinois at Urbana-Champaign aviation security expert Sheldon H. Jacobson agrees, saying the current method of airport screening is a waste of resources.
"We can actually look at research and discern the fact that a certain percentage of travelers and passengers just do not pose any threat to the system," Jacobson said in an interview with the university news bureau. "We have spent billions of dollars since 9/11, with a significant portion of that being spent to screen frequent business travelers, grandmothers and other passengers who pose no threat to the system. And if we use security resources on them, we're basically diverting away from the people who really require greater [scrutiny]."
Those billions went into setting up the Transportation Security Administration and buying and installing new equipment to scan both people and cargo. The equipment is sensitive enough that Homeland Security Secretary Janet Napolitano suggested last week we might be able to keep our shoes on pretty soon.
"We are safer from all the threats we know about. However, if we look at the trajectory of where aviation security is heading and the potential threats that could come onto the horizon, in many ways aviation security is at its riskiest and weakest point ever," Jacobson said.
"Every action and every layer of aviation security that has been added to the system has been based on a reaction to a previous threat. ...
"The one-size-fits all approach to passenger and baggage screening must be replaced with a system that directs technologies and procedures more appropriately. If we treat all the passengers the same, ultimately we're going to get to get a less-secure system.
"We don't want to search so much for the people who will pose a threat to the system, because that's like finding a needle in a haystack. What we really want to do is identify the relatively large group of people who pose no threat to the system -- that could represent 60 to 70 percent of all travelers. If we are able to break that group apart and subject them to a standard level of security, and use the available resources that they free up to screen the remaining 30 to 40 percent using these advanced technologies and procedures, then we will actually have a much more secure system."
Jacobson said it's the security holes of which we are still unaware that should worry travelers.
"People who are intent on causing harm to the air system are constantly testing the system, and with enough trial they will find these holes. The events of Sept. 11 were not a breach in aviation security; they were based on the security policies in place at that time and the problems with them," he said. "If we perpetuate that kind of activity and thinking, then we will continue to have problems.
"The focus has to be much more on allocating and using our scarce security resources in the optimal way so that travelers, when they get on an airplane, are going to feel more secure and more comfortable and know that their interests are being protected."
The Sept. 11 commission released a report card on improvements in U.S. security since the attacks and found though the nation is safer, there still are a number of shortcomings, including the failure to develop a biometric entry-exit system and lack of standardized identification.
"Our terrorist adversaries and the tactics and techniques they employ are evolving rapidly. We will see new attempts, and likely successful attacks," the commission report warned. "One of our major deficiencies before the 9/11 attacks was a failure by national security agencies to adapt quickly to new and different kinds of enemies. We must not make that mistake again."
'Happy' fan videos make Pharrell cry [VIDEO]
Pistorius testifies he didn't consciously pull trigger when he shot girlfriend