Federal air marshals are not on every one of the roughly 29,000 domestic and international flights that U.S. commercial carriers fly every day - and none was aboard Friday's Northwest Flight 253 to Detroit.
Instead, the Transportation Security Administration sends teams of undercover, plainclothes marshals with the Federal Air Marshal Service on certain routes that are deemed higher-risk because of threats, vulnerabilities and consequences - such as flights over potential high-profile targets, such as national landmarks.
Air marshals are armed, trained in hand-to-hand combat and learn how to shoot in cramped quarters. They are also trained to blend in with other passengers, the TSA says, while keeping an eye out for suspicious passenger behavior both on board and in the airport.
The exact number of air marshals serving today is classified, although a Government Accountability Office report from July puts the number in the "thousands." The program has greatly expanded since the 9/11 terrorist attacks, when fewer than 50 air marshals flew, and then only on international flights. The average marshal will fly five hours a day, 181 days per year, according to the TSA.
The Federal Air Marshal Service had its genesis in the Sky Marshal program of the 1960s and 1970s, which was created after a rash of hijackings of U.S. aircraft, according to the GAO.
The TSA would not provide additional information or offer a representative answers to questions about the air marshal program.