They fly undercover. Which planes they're on is a closely guarded secret. How many of them are employed by the government is classified.
Before the Sept. 11 attacks, the chances of meeting one on a flight were extremely slim: There were fewer than three dozen. Since then, the chances are much greater, with several thousand now flying, armed and anonymous.
Until Wednesday, no marshal had fired a weapon, though they had been involved in scores of incidents.
"That's a pretty remarkable record," said Rep. John Mica, R-Fla., who as chairman of the House aviation subcommittee was involved in the expansion of the air marshal service. "We've come from having a ragtag group to a highly trained air marshal team."
Rigoberto Alpizar, a 44-year-old American Airlines passenger, was shot and killed by an air marshal on a jetway after he ran from a Boeing 757 parked at Miami International Airport. Alpizar had claimed he had a bomb and failed to obey orders to stop, according to law enforcement officials.
Mica called it "an unfortunate incident."
"Everyone's on edge because we view the biggest threat as explosives, or bombs," he said.
The air marshal service began in 1968. President Reagan later expanded it, but by 9/11 the then-Federal Aviation Administration program consisted of just 33 marshals.
Thousands have been hired since then, though the overall figure is classified, and the service is part of the Transportation Security Administration.
When marshals fly, they attempt to blend in with other passengers. They don't identify themselves unless they have to do so.
The marshals train in New Jersey, learning to abide by their guiding principle: "Dominate. Intimidate. Control." They learn behavioral observation, intimidation tactics and self-defense. They also practice precision shooting, even though many come from police or military backgrounds.
Air marshals can be deployed on an hour's notice. In recent years, they have flown aboard every flight in and out of New Orleans during the Super Bowl, patrolled the skies above Salt Lake City during the Olympics and routinely fly wherever President Bush is.
James Carafano, a senior fellow at the Heritage Foundation in Washington, said the program is a key piece of of the stepped-up airline security measures put in place since Sept. 11.
"If you have the least amount of suspicion at all you can put an air marshal on there," Carafano said. "It allows you to do something other than nothing or canceling a flight."
Associated Press writer Leslie Miller contributed to this report.