Security experts say al Qaida and other terrorist networks are constantly trying to stay one step ahead of airport security measures. If the United States builds a wall, militants try to find a way around it.

Indeed, many security measures implemented since the 9/11 attacks -- from requiring passengers to remove shoes to restrictions on carrying liquids onboard -- in theory provide only temporary security.

That is because radicals are constantly seeking a loophole.

Body scanning technology, for example, cannot tell whether an explosive device is hidden in a body cavity, a fact that terrorists have used to their advantage.

Last year, radical operative Abdullah Asieri hid explosives and a detonator in his rectum in a bid to assassinate Prince Mohammed Bin Nayef, Saudi Arabia's counterterrorism chief. While the blast failed to kill the prince, it demonstrated the effectiveness of the method.H Others question whether body imaging equipment is worth the high price, and some experts noted that out of millions of flights and billions of flyers in the United States over the last decade, not one terrorist bomb has successfully exploded at a U.S. airport.

Some experts, however, argue that the new procedures adopted by the Transportation Security Administration (TSA), while no silver bullet, complicate terrorists' attempts to wreak havoc.

GO FOR THE BOMB, NOT BOMBER

Critics contend that authorities are searching for the bomb instead of the bomber. While technology has its place, it cannot identify suspicious behavior, as only humans have the right instincts to understand when someone is behaving suspiciously.

Despite the brainwashing before a suicide bombing, most bombers are under extreme stress while attempting to carry an explosive device on board an aircraft, which would cause them to exhibit some strange behavior.

Based on that, some experts advocate the Israeli approach, which employs highly trained agents to look for suspicious activity among passengers, although opponents counter that the method is impractical in the United States, given the high volume of daily air traffic.

On Wednesday, John Brennan, President Barack Obama's chief homeland security advisor, said U.S. security has improved since last year's attempted Christmas bombing.

"We are in a much better position today than we were last year at this time," he told reporters at the White House.

On Friday, Brennan convened an inter-agency coordination call with Homeland Security Secretary Janet Napolitano and other security chiefs, in which they reviewed the additional steps they said they are taking to ensure vigilance throughout the holiday season, including enhanced security measures and close coordination with foreign partners, according to the White House.

Some experts noted that terrorists sometimes win even when they do not conduct attacks, since their objective is to implant fear.

"To some extent terrorists win even when they fail," said Christopher A. Preble, director of foreign policy studies at the Cato Institute.