US fighter pilots are practising shooting down civilian airliners in the event of any new attack on America like the 11 September 2001 attacks.
Two hijacked civilian planes crashed into World Trade Center
"We practice this several times a week," Air Force General Ralph Eberhart, told reporters.
"[It could be] an airplane shooting down an airplane or air defence systems in the national capital region shooting down an airplane," he said.
General Eberhart said the pilots were being trained to overcome a natural hesitation to kill civilians.
Pilots and ground controllers are screened to make sure that they would not refuse an order to shoot down a suspicious airliner packed with civilians.
On 11 September 2001, four hijacked airplanes crashed into the World Trade Center in New York, the Pentagon outside Washington and a field in Pennsylvania.
About 3,000 people were killed in these attacks.
There is concern that the trained pilots may be "trigger hesitant" if given the order to shoot down a hijacked airline.
"Frankly, we have long discussions with people to see if they are mentally prepared to do this," General Eberhart said, adding it was not easy.
"You are up behind this airline that has hundreds of innocent people and now you have been told to shoot it down," General Eberhart said.
"That is much different than somebody you think is a threat to you who you are about to shoot."
General Eberhart said a careful chain of command and identification was in place to order such a shoot-down.
"Somebody just can't get up on the radio channel and say `Hey, I'm the President of the United States, shoot that down," he said.
General Eberhart said shooting down a suspicious civilian airline would be the "last resort".
"If we don't do this, innocent people on the ground are going to die."