Protecting air travel became a critical priority following the September 11th attacks.
So, 10 years ago this month, the new Transportation Security Administration took over aviation security. On the front lines, high above, are the federal air marshals.
Homeland security correspondent Bob Orr took a rare look at their training.
To make it as a federal air marshal, recruits must be able to run up evacuation slides, out-score other federal agents on gun ranges, and learn to react in life-or-death circumstances on crowded airplanes with no backup.
When it comes to finding new marshals, the TSA is looking for "someone who doesn't live in a black and white world, because at 35,000 feet we don't give them black and white answers," said Joseph D'Angelillio, head of air marshal training.
Prospects spend 120 hours training with weapons. Recruits also endure a rigorous mix of martial arts and mental exercises.
Since threats can emerge without warning, complacency is an enemy.
The program was languishing before 9/11. On the day of the attacks there were only 33 federal air marshals. Now there are thousands. The exact number is classified, but in the past two years alone, 25 new classes have gone through just one training facility.
The most recent graduates just got their badges from Homeland Security Secretary Janet Napolitano. Since the federal air marshals are America's most secret federal police force, we can't identify them.
Anonymity is critical, for air marshals to blend in with the 1.8 million people who fly each day.
"It's a game. It's been played since 9/11: Pick out the air marshals. And it is a great game because people assume they are everywhere," said D'Angelillio, adding that that assumption acts like "a force multiplier."
Air Marshal Kimberley Thompson flew for eight years before moving to headquarters. She says air marshals are taught to continuously scan for "potential" threats.
"We are looking continually at the passengers around us, the passengers that are getting up and going into the restrooms, or moving about the cabin for any given reason to determine why that person is getting out of their seat," Thompson said.
Despite the force build up, federal air marshals still cover just a fraction of the 30,000 daily flights in the U.S. Over the past decade, they've made few arrests, and have never fired a weapon in flight.
But, air marshals cannot lower their guard, knowing that aviation remains the top target of terrorists.