ARLINGTON, Va. — Lawmakers on Capitol Hill are demanding answers from the Transportation Security Administration (TSA) about a controversial program that targets airline passengers, including many who are not on any watch list.

Federal air marshals are trained to protect an airliner from a terrorist. But for the first time, the public is learning about another use of the air marshals program: the surveillance of some travelers. The TSA's "Quiet Skies" initiative started in 2010 and was expanded just last March.

The program, as first reported by the Boston Globe, identifies travelers, including American citizens, who could pose a threat, but may not have been accused of a crime and are not on the no-fly list. Undercover air marshals follow those passengers at an airport and on a flight, noting behavior like excessive fidgeting or perspiration, or having a "cold, penetrating stare."

Sources say the marshals observe up to 50 travelers a day on domestic flights. If no suspicious behavior is discovered people are removed from the program within 90 days. There is no electronic surveillance like wiretaps.

John Pistole, the president of Anderson University, was the TSA administrator when Quiet Skies launched.

"So the whole idea was, how can we mitigate risk against known risks and unknown risks? People who might do something but just haven't come up on anybody's radar yet," Pistole said.

Whistleblower Robert MacLean is an air marshal who was awarded protected whistleblower status after he raised concerns about agency practices in 2003. The TSA attempted to fire him, but in a case that went all the way to the Supreme Court, he was reinstated. He regularly tweets about his concerns over the lack of secondary barriers to block access to the cockpit when pilots enter or exit during flight.

"This goes back to what I say over and over, the air marshal's job is to protect the cockpit and the pilots," MacLean said. "Let somebody else do the intelligence and criminal investigative work."

In a statement, the TSA said: "The primary purpose of this program is to ensure passengers and flight crew are protected during air travel. Contrary to the article 'Welcome to the Quiet Skies' published by The Boston Globe, the program doesn't take into account race and religion, and is not intended to surveil ordinary Americans. In the world of law enforcement, this program's core design is no different than putting a police officer on a beat where intelligence and other information presents the need for watch and deterrence. The program analyzes information on a passenger's travel patterns, and through a system of checks and balances, to include robust oversight, the program effectively adds an additional line of defense to aviation security. With routine reviews and active management via legal, privacy and civil rights and liberties offices, the program is a practical method of keeping another act of terrorism from occurring at 30,000 feet."