ATLANTIC CITY, New Jersey When you think of aviation security, you're likely to conjure up images of security screeners at TSA checkpoints.

But the Federal Air Marshal Service is a clandestine layer which operates in plain sight every day.

With a requirement to be accurate at least 85 percent of the time, no other federal agent has sharper shooting skills than air marshals.

Our main work environment is the aircraft, air marshal Kimberley Thompson explained. At 35,000 feet, you don't have room for error.

For decades, the covert organization has avoided cameras. But it agreed to give us a rare inside look at its training facility in Atlantic City, New Jersey along with two former Dallas police officers who are rising through the ranks.

Tony Metcalf carried Badge 6666 with DPD. He worked downtown and as a DWI officer before 9/11 and FAMS began recruiting.

I faxed a two-page resume and got a call the next morning, Metcalf said. I actually thought it was a co-worker of mine playing a joke.

Metcalf said air marshals develop cover stories to explain why they're flying in case other passengers strike up a conversation.

Blending in is paramount, Metcalf said, and that's much easier for his female colleagues like Thompson.

I think the only unique challenge for women is the fact that the weapon is a pretty large weapon, and sometimes it is hard to determine what you need to wear to conceal it, she said.

Thompson spent four years as a Dallas police officer working at Northwest Patrol.

When 9/11 happened, it was something that spoke to me and said I need to do something more, she said.

Thompson and Metcalf are both supervisors now and no longer fly undercover, which is why News 8 revealed their images and identities.

Four times a year, every air marshal must prove his or her expertise.

From simulated aircraft seats positioned on an indoor range, air marshals run through exercises where they pick off paper threats with surgical precision.

In our environment, on an aircraft, we can't afford to miss, Metcalf said. Accuracy is very important.

Though they wouldn't reveal specific tactics of training, marshals slide down in their seat and draw their weapon before taking aim and pulling the trigger.

Marshals also learn close quarters combat, how to disarm attackers, and even certain cockpit controls.

If there ever was an emergency where we were asked by the crew to help out, we at least know where some of the major controls are including communicating with Air Traffic Control, Thompson explained.

While the official number is closely guarded, there are believed to be about 4,000 air marshals. That's far too few to protect up to 30,000 commercial flights in the United States every day.

Because of that, marshals say they operate with a threat-based mentality and focus on specific destinations or flights facing threats.

We may be on some flights. We may not be, Metcalf said. I think keeping the terrorists in a guessing game is very important.

The further the country gets from September 11, 2001, some argue the fewer air marshals are needed.

No one knows how many arrests they make; that, too, is an operational secret.

Plus, the service costs taxpayers almost a billion dollars a year. All told, that has led some in Congress to question whether it's still worth it.

This money is a total waste, Rep. John J. Duncan (R-Tennessee) told the House of Representatives in 2009. $860 million for people to sit on airplanes and simply fly back-and-forth, back-and-forth. What a cushy, easy job.

Critics like Duncan also complain that because the air marshal service is so secretive, taxpayers have no way to measure the effectiveness of such a large public investment.

Law enforcement officers aren't just out there to make arrests, Thompson countered. Law enforcement officers are there to be a deterrent. And we've been an effective deterrent so far.

Indeed, there hasn't been another deadly attack on a U.S. airliner since 2001.

The TSA clearly deserves some of that credit. Not just security screeners, but also the elite Federal Air Marshals, who are likely to be the last line of defense against another 9/11.

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