The 9/11 attacks, the deadliest terrorist acts on U.S. soil, were the first to use airliners as weapons. More attempts followed, including Richard Reid's attempt in December 2001 to ignite explosives in his shoes on a flight from Paris to Miami, and at  least 10 airliners were targets in a plot involving liquid explosives in 2006, forcing new policies regarding shoes at screening and liquids on airplanes.  Then, on Christmas Day in 2009, Umar Farouk Abdulmutallab tried to detonate explosives in his underwear on a flight from Amsterdam to Detroit.

Each of these failed attempts to bring down airliners helped spur the adoption of new passenger screening techniques and technologies, especially in the U.S. In response to Abdulmutallab, for example, the Transportation Security Administration (TSA) introduced about 500 advanced imaging technology units in December 2010, all of which scan ned bodies without physical contact. These include millimeter wave technology, which uses millimeter wavelength radio waves to generate three-dimensional images as well as backscatter scanners, which employ low-level x-rays. A host of complaints over how such technologies can invade privacy have led to plans to revamp these scanners, replacing images of passengers with generic outlines.

Before the 9/11 attacks, passenger screening involved metal detectors, but these were closer to the gates than they are now, "making it easier to stash or cache weapons such as the box cutters [the 9/11 attackers] used," says Joshua Marpet,  security evangelist at security firm DataDevastation. "Now the checkpoints act as choke points, severely limiting the availability of stash spots."

» View a slide show on the history of airport screening techniques and technology

The advanced imaging technologies now used in U.S. airports would catch anyone using the same strategies the 9/11 terrorists did—"but realize nothing, and I mean nothing, is as important as trained professionals watching for something out of place," Marpet notes. "Technology assists the agent in his or her job. It is not the be-all, end-all."

A determined opponent can often find ways around technology. "Metal detector? Plastic knife—Grivory is a popular plastic, fiberglass-reinforced plastic, actually, used by Cold Steel to make nonmetallic knives," Marpet says. But might such a knife get caught on millimeter wave scanners? "Not if it's buried inside flesh, say, in a particular cavity."

People, on the other hand, often inadvertently give away clues. "You could have a fanatic who just doesn't care, or honestly wants to die—they might have good control of their reactions and not look out of place, but they will be, by definition, a fanatic, and that will cue inappropriate or 'out-of-place' behaviors," Marpet says.

So what use then are these new technologies? "Deterrence," Marpet says. "Crooks and terrorists go after the low-hanging fruit, the easy target...if the security technology and agents can make it more expensive in time, effort, expense to go after planes, terrorists will pick other targets."

One intriguing new passenger screening technology is terahertz scanning. These use terahertz rays, which lie between microwaves and infrared radiation on the electromagnetic spectrum. They  can penetrate common materials, do not seem to harm living tissue and can identify compounds, such as hair gel or explosives.

"The ability to see the density of objects is awesome, and the use of that technology can be vastly more discerning as to weapon or pen or whatever than we have now," Marpet says. "I've seen plastic razors with it, and I don't think backscatter would even have touched it."