From the History Commons Groups blog:

9/11 Training Exercise Planned for Simulated Plane Crash Five Minutes before Pentagon Attack Took Place

Five minutes before the Pentagon was hit on September 11, 2001, a training exercise being run by a US intelligence agency just over 20 miles from the Pentagon was set to include the scenario of a small private jet plane crashing into a building. It is unclear whether the scenario was played out, or if the exercise had been called off by that time.

Important details of the exercise, which was being conducted by the National Reconnaissance Office (NRO) at its headquarters in Chantilly, Virginia, are revealed in a document obtained by the 9/11 Commission. The document, titled "Early Morning Flight Activity September 11, 2001," was part of a series of 9/11 Commission records moved to the US National Archives at the start of this year. It was found there, and posted online, by History Commons contributor paxvector.

Exercise Observers Meet at 9:00 a.m.

The NRO exercise, which had been planned for several months, was set to commence at 9:00 a.m. on September 11, when its observers would meet to be briefed. The observers and exercise role players were to move to their positions for the exercise 10 or 15 minutes later. In the exercise scenario, a Learjet 35A with two pilots and four passengers on board would take off at 9:30 a.m. from Washington Dulles International Airport. This airport, which is located four miles from the NRO headquarters, is where American Airlines Flight 77--the plane that reportedly hit the Pentagon--took off from earlier that morning.

About a minute after the Learjet took off, an explosion would be heard, and the pilot would complain that one of the engines was on fire and he was losing altitude. Around 9:32 a.m., the plane would crash into tower 4 at the NRO headquarters. Since the Pentagon was hit at 9:37 a.m., this means the crash in the scenario was scheduled to occur just five minutes before the actual attack occurred at the Pentagon, which is 24 miles away from the NRO headquarters.

Several People Killed and Injured in Scenario

The Associated Press has revealed that no real plane was going to be used in the exercise, and the crash was to be the result of mechanical failure, not terrorism. But the consequences of the simulated crash would be similar to those of the actual 9/11 attacks, albeit on a smaller scale. The newly released document describes the scene: "Various parts of the aircraft struck the outside portions of the building, spraying jet fuel. The final portions of the wreckage were scattered around the entryway between tower 1 and 2. Jet fuel was burning uncontrollably in the vicinity of the flagpoles. There are a number of injured and dead NRO employees." Some stairwells and exits at the NRO headquarters were going to be closed off in order to simulate the damage from the crash, thereby forcing employees to find other ways to evacuate their building.

Exercise 'Inputs'

The document reveals that the exercise was set to include numerous "inputs," which appear to have been communications and other actions intended to make it appear more realistic to its participants.

Planned inputs included, at 9:30 a.m. a smoke generator was going to be started, to simulate the fire resulting from the crash. At 9:32, numerous phone calls would begin flooding in, from people reporting fires in various locations in the building. At 9:34, after someone reported that a small civilian jet had crashed, NRO personnel were to be instructed to evacuate their building.

At 9:37, the first engine from Fairfax County Fire Department was scheduled to arrive on the scene. (It is unclear whether real fire department personnel were going to participate in the exercise. The document states that "inputs from simulated Fairfax responders" were to be used "if Fairfax does not play.") At 10:03, four more fire department trucks and emergency medical technician vehicles would respond to the crash. By 10:30 all the simulated fires would have been put out, but it would be confirmed that at least four NRO employees died in the crash. The exercise was set to end at 11:45 a.m.

Exercise Canceled

The exercise was reportedly called off in response to the morning's real world attacks. However, the specific time when it was canceled is unclear. NRO spokesman Art Haubold has said only that "as soon as the real world events began, we canceled the exercise." It is therefore unknown whether the simulated plane crash was played out, or whether the exercise had been brought to an end before it took place. After the exercise was canceled, all but the NRO's most essential employees were sent home.

The Highly Secretive National Reconnaissance Office

The National Reconnaissance Office is a highly secretive organization. The New York Times called it "probably the most secretive of the intelligence agencies." Until 1992, its existence had not even been officially disclosed. It is an agency of the US Department of Defense, and is responsible for ensuring "that the US has the technology and spaceborne and airborne assets needed to acquire intelligence worldwide." According to the New York Times, the NRO "designs, builds, and operates spy satellites that photograph and overhear what other countries are up to." It employs some 3,000 people, who are drawn from the CIA and the military.

Source:
http://hcgroups.wordpress.com/2009/09/07/911-training-exercise-planned-for-simulated-plane-crash-five-minutes-before-pentagon-attack-took-place/