If fences and barriers are the first line of defense, the air marshals are the last. If everything else fails and a terrorist still gets onto a flight with a weapon, an armed air marshal can take control of a situation and restrain the attackers. Although the air marshal program has existed since the 1970s, it has never had as high of a profile as it has in the post-9/11 era.
An air marshal is a federal agent disguised to look like regular passenger. Each air marshal is authorized to carry a gun and make arrests. There are not enough air marshals to cover every flight, so their assignments are kept secret. No one knows which passenger is the air marshal, or even if an air marshal is present on the flight at all. Although their exact numbers are kept classified, airline insiders estimate that only five percent of U.S. flights have an air marshal on board. This is still a major increase - in the years before 9/11, a handful of marshals guarded just a few international flights.
In addition to policing the sky, new laws have forced the installation of locks on cockpit doors. This could prevent hijackings by terrorists who are trained to fly passenger jets by keeping them away from the plane's controls.
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