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As the head of the Transportation Security Administration answered questions on Capitol Hill on Wednesday about a controversial program that monitors air passengers, an undercover air marshal defended the practice as necessary work in an age of terror threats.

"My mission every day is to work the flight that I am assigned and make sure it goes up wheels up to wheels down without incident," the air marshal, who spoke on condition of anonymity, told NBC News.

"And that doesn't just start on the aircraft — that starts in the airport from observing my surroundings, understanding what's normal, attempt to pick out any abnormalities and then nullify them as they present themselves," the air marshal, who has been on the job for eight years, said.

The TSA's "Quiet Skies" program has alarmed some privacy advocates who say that ordinary Americans are being surveilled, and that those not accused of crimes or suspected of having ties to terrorism should not be caught up in the program, which the TSA chief said has been in place since 2011.

The previously undisclosed program was exposed by the Boston Globe in July, which reported that the program tracks U.S. citizens who have been flagged to the TSA based on their affiliations or travel histories, even if they are not suspected of a crime or on any terrorist watch list.

TSA agents check passenger boarding passes and identification at a security screening checkpoint, at Seattle-Tacoma International Airport in Seattle on May 19, 2016. Ted S. Warren / AP file

Two to three dozen travelers are followed each day. People attending craft shows, flight attendants, and even law enforcement officers have been monitored. Marshals take note of what a traveler reads, who they talk to, and even whether they use the restroom.

TSA Administrator David Pekoske on Capitol Hill on Wednesday acknowledged that air marshals operating under the program have made no arrests or foiled any known plots. Last month, the Homeland Security Office of the Inspector General announced it would investigate the program.

"I am confident that it has reduced risk for nearly seven years and I would note that we are fortunate to have the air marshal capability supporting this program," Pekoske told lawmakers Wednesday.

"There is, I assure you, strong oversight of this program from the department," Pekoske said. He said much of the program involves security-sensitive information, and that the intelligence that supports it is classified.

The air marshal who spoke with NBC News said that "it's not any different than putting a police officer on the street in potentially a high crime area in order to deter that crime from taking place."

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More than 700 million passengers flew on domestic flights in the U.S. in 2017, according to the federal Bureau of Transportation Statistics.

Sen. Ed Markey, D-Mass., said at Wednesday's hearing before the Senate Commerce, Science and Transportation Committee that "the American public does deserve to know the extent to which they are being surveilled."

The air marshal said that the program identified people "that have certain travel patterns and uses other information to enroll them in this program."

"The program essentially gives a little bit of information about certain individuals when they travel and when we deem it appropriate, we will staff those flights with air marshals and we'll be able to observe behaviors of this individual as well as behaviors of other passengers," the air marshal said.

It's not just privacy advocates who have raised concerns about the program. The Globe reported that dozens of air marshals have raised concerns about the Quiet Skies program with senior officials and colleagues, sought legal counsel, and expressed misgivings about the surveillance program.

"What we are doing [in Quiet Skies] is troubling and raising some serious questions as to the validity and legality of what we are doing and how we are doing it," one air marshal wrote in a text message to colleagues, according to the Globe.

The air marshal said marshals do not determine who is put into the program and that "under no circumstances does the Federal Air Marshals Service, to include my colleagues and myself, utilize race or religion to profile passengers, period."

Air marshals look for "abnormalities in behaviors" from everyone on the plane, including those flagged under the Quiet Skies program.

That could include someone nervous or who is fidgeting, or having a conversation, but he would not go into specifics of what air marshals do look for or consider suspicious.

"Sometimes it's something as simple as a casual conversation, and if things progress from there then that's something we turn over to the local law enforcement entity," the air marshal said.

The air marshal said that the system is designed to remove people from the program who do not arouse suspicion as quickly as possible, describing the task of watching for potential threats like finding a needle in a haystack.

"We're interested in shrinking the size of that haystack, not increasing it," the air marshal said.

The head of the Transportation Security Administration David P. Pekoske, center, talks during a news conference in Los Angeles' Union Station on Aug. 14, 2018. Richard Vogel / AP file

The TSA has described Quiet Skies as a "practical method of keeping another act of terrorism from occurring at 30,000 feet."

Pekoske said Wednesday that once people are determined to not pose a risk, they are removed from the Quiet Skies program and those records are retained for two years.

After Markey said that there has been no evidence that any attacks have been prevented under the program, Pekoske said "there is evidence that it has transitioned some passengers identified in Quiet Skies to the known or suspected terrorist list."

"It is successful," he said.

But some legal experts have said that the agency has not demonstrated any need for the program, or whether it is effective.

Hugh Handeyside, senior staff attorney with the American Civil Liberties Union's National Security Project, told NBC News in July that "the whole thing is just absurd on so many levels."

The TSA chief said Wednesday that thousands of travelers have been followed since March. While nearly all were cleared, some were placed on the terrorist watchlist.