Federal air marshals have for years been quietly monitoring small numbers of U.S. air passengers and reporting on in-flight behavior considered suspicious, even if those individuals have no known terrorism links, the Transportation Security Administration said on Sunday.

Under a sensitive, previously undisclosed program called "Quiet Skies," the TSA has since 2010 tasked marshals to identify passengers who raise flags because of travel histories or other factors and conduct secret observations of their actions — including behavior as common as sweating heavily or using the restroom repeatedly — as they fly between U.S. destinations.