In the past decade, the number of U.S. air marshals has increased exponentially, from 33 to an estimated 3,000 to 4,000 agents.
But while there's a one in 20 chance that your flight has an air marshal aboard, you're unlikely to realize it: Marshals wear regular clothing to "blend in with passengers," according to the TSA. Their training is also tailored to aircraft-specific situations, incorporating investigative techniques, behavior recognition and self-defense training designed for the closer quarters of an airplane.
The air marshal program began in 1962, after former President John F. Kennedy signed an amendment to the Federal Aviation Act of 1958. The amendment enhanced the penalty for interfering with airline crew members, according to the TSA.
When X-ray screening was introduced at airports in 1974, the number of air marshals greatly decreased. That, of course, changed after Sept. 11, 2001. At the time of the 9/11 attacks, only 33 agents were in the air marshal program.
An infographic from Instant Checkmate has more about air marshals and the history of the program.