Most Americans have never heard of the Federal Air Marshals, and the government likes it that way. But the airborne attacks on the World Trade Center and the Pentagon have led to promises from the Bush administration to expand the shadowy force, despite questions about whether they really deter terrorism.

Started by President Nixon in 1970, the air marshal team is the Federal Aviation Administration's unit of high-tech sharpshooters. Dressed in civilian clothes, they board flights at random or in response to specific threats, carrying guns and special ammunition that is designed to kill without damaging the airplane.

''They have some of the highest, if not the highest, firearms qualifications in the federal government,'' FAA spokeswoman Rebecca Trexler said. ''They don't miss.''

Last week, Rep. John Mica, the chairman of the House Transportation aviation subcommittee, urged expansion of the program during meetings with the attorney general and the transportation secretary, said his spokesman, Gary Burns.

''He expressed his views to the administration that the sky marshal program should be strengthened and used to its full effectiveness to give passengers a fighting chance when necessary,'' Mr. Burns said.

The Transportation Department requested a ''substantial increase'' in the air marshal program as part of the $40 billion emergency spending bill passed by Congress on Friday, department spokesman Bill Mosley said.

The Air Transport Association, the trade organization for U.S. airlines, also called for more air marshals last week.

The FAA has always been secretive about the air marshals, refusing to divulge their number, how they work or what they look like. The assault teams wear masks when giving rare public demonstrations.

''We don't want to give the bad guys any clue about what their chances are,'' Ms. Trexler said.

Agents in the 1970s were called sky marshals and were part of the Customs Service. The current FAA-led force was formed after the hijacking of TWA Flight 847 in June 1985.

At their base near Atlantic City, N.J., the marshals practice in two retired airliners. They also have a five-story simulated control tower, three outdoor shooting ranges with moving targets and a high-tech ''shoothouse'' with a mock-up airplane inside.

The marshals shoot hollow-point, aluminum bullets that kill without penetrating the skin of airplanes. Each agent spends at least three hours a week in shooting practice.

But the force has plenty of skeptics.

Several times, marshals have helped subdue passengers with so-called ''air rage'' - but as far as anyone knows, they have never gone up against a team of armed, well-trained terrorists while in flight, said Richard Bloom, the director of terrorism and intelligence studies at Embry-Riddle Aeronautical University in Prescott, Ariz.

''They are most effective against mentally deranged people, people working alone who threaten a flight,'' Mr. Bloom said. ''When you're looking at sophisticated, coordinated planning, there's not much an air marshal can do.''

Undercover agents are also helpless against bombs, which previously caused far more deaths than hijackers, critics say. And there is always the risk that a gun battle at 30,000 feet could rupture hydraulic lines or kill a pilot.

''We just don't know if (the marshals) are effective; they've never been tested,'' said Charles Slepian, an aviation security consultant in New York. ''If it's Wesley Snipes in a movie, I know he can shoot the gun right out of the hand of the hijacker. But a real sky marshal? I have no idea.''

Despite doubts over the marshals' effectiveness, most experts predicted there will soon be many more of them as a result of Tuesday's hijackings.

''People used to say it was more dangerous to have air marshals in the air when in many cases a hijacker can be talked down instead,'' said Robert Blair, an international security consultant based in Pensacola, Fla. ''But if these (hijackers) are planning to turn the aircraft itself into a weapon, they're not going to be talked down.''