A Democratic senator is raising concerns over reports of a previously secretive federal program that monitors traveler behavior, questioning whether the initiative infringes on individuals' privacy.

Sen. Ed Markey Edward (Ed) John Markey3 reasons why Biden is misreading the politics of court packing Democrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement MORE (D-Mass.) sent a letter to the head of the Transportation Security Administration (TSA) on Monday following a Boston Globe report on the "Quiet Skies" program, under which undercover federal air marshals observe behavior by passengers.

Markey wrote that the program "raises serious privacy concerns" and posed several questions to the TSA about how individuals are selected for the program as well as how effective it has been in the years since it was implemented.

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“The Quiet Skies program is the very definition of ‘Big Brother’, and innocent Americans should not be subject to this kind of violation of their rights,” Markey said in a statement.

He asked TSA Administrator David Pekoske if anyone monitored through the Quiet Skies program has been arrested or prosecuted, if the initiative has uncovered any threats to an airplane or the broader public, and what means of surveillance the TSA uses for the effort.

Markey requested answers by Aug. 20.

The TSA has drawn criticism at times for its intrusive security measures, and lawmakers have scrutinized the effectiveness of the federal air marshal program.

Lawmakers and privacy experts have also expressed specific concerns about the Quiet Skies program in recent days, saying it could cross a line if it's too selective in who it targets.

TSA officials have disputed, however, that race or religion is a factor in who is selected for the program.

Michael Bilello, an assistant administrator of public affairs for TSA, pushed back against those criticisms, telling The Hill on Monday that Quiet Skies is managed, in part, by "legal experts, privacy experts and civil liberties experts."

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

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