The Transportation Security Administration (TSA) is defending its use of a previously secret program used to monitor potential security threats at airports and on planes.

Michael Bilello, an assistant administrator of public affairs for TSA, told The Hill that the “Quiet Skies” initiative does not focus on “ordinary Americans,” but instead zeroes in on a small percentage of the traveling population.

“These programs are not designed to observe the average American,” he said. "They’re designed to protect the traveling public, but they’re not targeting the average American.”

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"We’re talking about a very unique passenger that warrants the attention of a federal air marshal," he added.

The Boston Globe first reported on Sunday that the TSA implemented the “Quiet Skies” program years ago to help eliminate threats posed by “unknown or partially known terrorists.”

Through the program, undercover federal air marshals observe passengers’ behavior. This can include watching how close they stand to the boarding area, how often they use the bathroom and any behavioral tics such as sweating or twitching.

The report included critical accounts of the program from some air marshals, who said they felt it was a poor use of resources to track nonthreatening travelers.

Bilello on Monday was able to provide few specifics on how a traveler ends up being monitored through the program, but disputed the idea that it constitutes “surveillance,” arguing it consists solely of visual observation.

He also defended its role in keeping the skies safe.

The program is aimed at travelers who display “travel patterns that will initially flag them as somebody that should receive further analysis,” Bilello said.

TSA then analyzes additional information available through law enforcement agencies and other databases, he said.

“If after that completion of analysis that person is deemed a potential threat to aviation, the flight crew, passengers, they could be entered into the program and be assigned a federal air marshal for observation,” Bilello said.

If an individual does not act on any suspicions over a certain period of time, Bilello said, they are dropped from the Quiet Skies program.

The TSA has drawn criticism at times for its intrusive security measures, and lawmakers have scrutinized the effectiveness of the air marshal program. As The Boston Globe’s report made the rounds on Sunday and Monday, some privacy experts expressed concerns that the Quiet Skies initiative could cross a line if it is too selective in who it targets.

Hugh Handeyside, an attorney with the American Civil Liberties Union’s National Security Project, told The Washington Post that the TSA should provide more information on the program. He criticized the initiative as a “waste of taxpayer money."

Bilello pushed back against those criticisms, saying the Quiet Skies effort is managed, in part, by "legal experts, privacy experts and civil liberties experts."

He also defended the role of the air marshal program, calling it "necessary if we’re going to avoid another 9/11."

TSA officials have also insisted that the program does not target people based on their race or nationality, something TSA has sometimes come under criticism for doing.