Next, secondary screening can involve hours of questioning. Agents have been known to click through all of a traveler's digital photographs. Body searches are common, and agents usually take luggage apart one item at a time.   

In addition, officials think of passengers as passing through a series of concentric circles, with increasing scrutiny as they get closer to boarding the plane. Agents also pay close attention to the parts of the airport that passengers don't frequent. They monitor the fences around the airport's perimeter with cameras at all times, and radar systems check for intrusions when the weather prevents the cameras from seeing. Security officials subject all vehicles to a weight sensor, a trunk x-ray, and an undercarriage scan.

According to a Slate magazine report, Israeli researchers are developing technology that could ease racial profiling concerns, like check-in kiosks to replace the human selectors. When a traveler steps up to the machine, it senses his body temperature, blood pressure and heart rate. At some point during the interaction, the kiosk presents a statement that would elicit a reaction from a would-be terrorist, the report says. It might instruct him to see an agent, or just remind the passenger that flight security is everyone's responsibility. If the flyer's vital signs shift, he would be subject to secondary screening. But while officials in the U.S., Europe, and Canada are considering the high-tech solution, Israeli officials haven't shown much interest, says the report. They think that security risks at Israeli airports require human profilers.