The Infinite Brief

The previously undisclosed program, called “Quiet Skies,” specifically targets travelers who “are not under investigation by any agency and are not in the Terrorist Screening Data Base,” according to a Transportation Security Administration bulletin in March.

The internal bulletin describes the program’s goal as thwarting threats to commercial aircraft “posed by unknown or partially known terrorists,” and gives the agency broad discretion over which air travelers to focus on and how closely they are tracked.

TSA officials, in a written statement to the Globe, broadly defended the agency’s efforts to deter potential acts of terror. But the agency declined to discuss whether Quiet Skies has intercepted any threats, or even to confirm that the program exists.

Already under Quiet Skies, thousands of unsuspecting Americans have been subjected to targeted airport and in-flight surveillance, carried out by small teams of armed, undercover air marshals, government documents show. The teams document whether passengers fidget, use a computer, have a “jump” in their Adam’s apple or a “cold penetrating stare,” among other behaviors, according to the records.