"Obviously, aviation is our primary mission, and since 9/11, we've had tens of thousands of flights that have flown successfully under our watchful eye," she said.

While it's a common belief that an air marshal's job consists only of sitting on a plane and flying back and forth across the country, that is a misperception, Norris said.

"We're there to ensure the safety and security of the traveling public, so it's our vigilance and our training that allows us to be ready to react should there be any type of threat or situation on a plane that could harm the passengers, the crew, the aircraft," she said.

The congressman, however, said arrest records show that air marshals do very little. Since 2001, the entire agency has averaged slightly over four arrests per year. That comes to about one a year per 1,000 employees and means the government is spending about $200 million per arrest, Duncan said.

"When we are so many trillions of dollars in debt -- a national debt of over $13 trillion -- we simply cannot afford to waste money in this way," he said.

Norris countered that arrests aren't a good way to measure the agency's success. Federal air marshals are trained to report suspicious activity to local law enforcement agencies, "so even in situations where an arrest would be appropriate, the majority of that is handled by the local law enforcement in whatever airport or location we are at," she said.

Duncan also pointed to news reports that said dozens of air marshals have been charged with crimes or accused of misconduct since 9/11, including drunken driving, domestic violence, human trafficking and attempting to smuggle explosives from Afghanistan.