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.

Ardent Sentry Tests U.S., Canadian Crisis Response Capabilities

By Gerry J. Gilmore, American Forces Press Service
Apr 29, 2007 - 5:39:47 PM

Blackanthem Military News, WASHINGTON, D.C. – Thousands of active-duty and National Guard servicemembers will take part in a two-week, Defense Department-sponsored nationwide emergency preparedness and response exercise that kicks off April 30, a senior department official said here yesterday.

A major focus of Operation Ardent Sentry - Northern Edge 2007 will be to test crisis-response coordination between federally controlled military forces and National Guard units that come under the command of state governors, Peter F. Verga, acting assistant secretary of defense for homeland defense, told the Pentagon Channel and American Forces Press Service at the Pentagon.