10 Years Later: 9/11 Security Then and Now

Former TSA head warns of "failure of imagination" when predicting terrorists.

Sept. 8, 2011 — -- While air travel security may look vastly different now than it did before the Sept. 11, 2001 terror attacks, one former TSA administrator said that he's afraid not of a single attack, but of a "failure of imagination" on the part of U.S. security officials going forward.

"I think that's the most haunting thing, failure of imagination," former TSA administrator Kip Hawley told ABC News. "If you can't imagine it... How do you stop it?"

WATCH: Ten Years After 9/11, Are We Safer?

A recent report by the Bipartisan Policy Center said that U.S. security has succeeded in implementing some of the 9/11 Commission recommendations, but noted that the country has failed in significant areas as well.

READ: Sept. 11 Anniversary Report Card: Not a Lot of A's

"In 2001, security was illusory. The airlines pretended to deliver security and the government pretended to find it OK," former Department of Transportation Inspector General Mary Schiavo said. "Overall, we're better [now]... but we still have a long way to go."

After the TSA took over airport security from the airlines after the Sept. 11 attacks, it implemented significant changes to the passenger screening process -- but often only in reaction to failed terror plots that had slipped under the radar.

Passengers flying today have to remove their shoes when going through security because nearly 10 years ago convicted terrorist Richard Reid managed to sneak explosives on a trans-Atlantic flight in his shoes. Luckily Reid was not able to detonate the shoes.

READ: Napolitano's Future: Airline Passengers Can Keep Shoes On

All liquids brought aboard a plane are now restricted to small, transparent containers, but only after British investigators foiled a plot by three British men to kill fellow passengers on flights from the U.K. to the U.S. using explosives made from concentrated hydrogen peroxide smuggled aboard in sports bottles in 2006.

"They found a formula that the bomb experts did not think was possible," Hawley said.

READ: Inside the Liquid Bomb Plot

Hawley said that because of the TSA's efforts, shoe bombs and bottle bombs -- "two classes of explosives that are most dangerous" -- are off the table for terrorists, leading them to consider less reliable options.

"It forces them to the fringes of using bombs that may or may not work," he said. "Yes, you don't want a bomb going off and injuring people on a plane, but you do not want to let them bring on a bomb that will catastrophically destroy the plane."

Apart from bombs, U.S. officials told ABC News they believe fortified cockpit doors and armed pilots and air marshals make another hijacking highly unlikely. But Hawley said that as rigid as security measures must be to stop plots like those seen before, they have to be flexible enough to predict new ways terrorists could strike.

"I think you have to be aware of all the possibilities and have security that is flexible and smart," he said, "even to find plots you can't imagine."