WASHINGTON, April 13— At least five months before the attacks of Sept. 11, 2001, air defense planners proposed a war-game situation in which a terrorist group hijacked an airliner and flew it into the Pentagon, but the Joint Staff rejected the idea, Pentagon officials said Tuesday.
Pentagon officials on Tuesday played down the significance of the proposal, which they said was made at a planning conference before the training exercise in April 2001, and was not based on any specific threat. The exercise was intended to test the military leadership's ability to function at undisclosed sites outside Washington in an emergency, and the hijacking did not fit the war game's objectives, the officials said.
''Several fictitious scenarios were posed, and this scenario was rejected, as were many others,'' said Bryan G. Whitman, a Pentagon spokesman.
The proposal was described in an e-mail message disclosed on Monday by the Project on Government Oversight, an advocacy group, and was confirmed by military officials. The message was written Sept. 18, 2001, by Terry Ropes, identified as a former member of the North American Aerospace Defense Command, or Norad, which is responsible for defending United States airspace.
''The Norad exercise developers wanted an event having a terrorist group hijack a commercial airliner (foreign carrier) and fly it into the Pentagon,'' the message said. ''Joint Staff action officers rejected it as unrealistic.''
In April 2001, a Pentagon official said, ''it probably was considered unrealistic.'' He added, ''While Norad had done hijacking scenarios, they normally landed safely and resulted in negotiations.''
The message also said that the Navy's Pacific Command also balked at using the hijacking, but a spokesman for the command said it had never taken a position on it.
An investigator for the advocacy group, Peter Stockton, said the hijacking proposal was suggested by Special Operations personnel trained to think like terrorists.
