From The Boston Globe story here:
Quiet Skies represents a major departure for TSA. Since the Sept. 11 attacks, the agency has traditionally placed armed air marshals on routes it considered potentially higher risk, or on flights with a passenger on a terrorist watch list. Deploying air marshals to gather intelligence on civilians not on a terrorist watch list is a new assignment, one that some air marshals say goes beyond the mandate of the US Federal Air Marshal Service. Some also worry that such domestic surveillance might be illegal. Between 2,000 and 3,000 men and women, so-called flying FAMs, work the skies.
Since this initiative launched in March, 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, according to interviews and documents reviewed by the Globe. ...
Agency documents show there are about 40 to 50 Quiet Skies passengers on domestic flights each day. On average, air marshals follow and surveil about 35 of them.