WASHINGTON — In what the government describes as a bizarre coincidence, one U.S. intelligence agency was planning an exercise last Sept. 11 in which an errant aircraft would crash into one of its buildings. But the cause wasn't terrorism -- it was to be a simulated accident.

Officials at the Chantilly, Va.-based National Reconnaissance Office had scheduled an exercise that morning in which a small corporate jet would crash into one of the four towers at the agency's headquarters building after experiencing a mechanical failure.

The agency is about four miles from the runways of Washington Dulles International Airport.

Agency chiefs came up with the scenario to test employees' ability to respond to a disaster, said spokesman Art Haubold. No actual plane was to be involved -- to simulate the damage from the crash, some stairwells and exits were to be closed off, forcing employees to find other ways to evacuate the building.

"It was just an incredible coincidence that this happened to involve an aircraft crashing into our facility," Haubold said. "As soon as the real world events began, we canceled the exercise."

Terrorism was to play no role in the exercise, which had been planned for several months, he said.

Adding to the coincidence, American Airlines Flight 77 -- the Boeing 767 that was hijacked and crashed into the Pentagon -- took off from Dulles at 8:10 a.m. on Sept. 11, 50 minutes before the exercise was to begin. It struck the Pentagon around 9:40 a.m., killing 64 aboard the plane and 125 on the ground.

The National Reconnaissance Office operates many of the nation's spy satellites. It draws its personnel from the military and the CIA.

After the Sept. 11 attacks, most of the 3,000 people who work at agency headquarters were sent home, save for some essential personnel, Haubold said.

An announcement for an upcoming homeland security conference in Chicago first noted the exercise.

In a promotion for speaker John Fulton, a CIA officer assigned as chief of NRO's strategic gaming division, the announcement says, "On the morning of September 11th 2001, Mr. Fulton and his team ... were running a pre-planned simulation to explore the emergency response issues that would be created if a plane were to strike a building. Little did they know that the scenario would come true in a dramatic way that day."

The conference is being run by the National Law Enforcement and Security Institute.