WASHINGTON (CNN) -- Sometime between 1991 and 2001, a regional sector of the North American Aerospace Defense Command simulated a foreign hijacked airliner crashing into a building in the United States as part of training exercise scenario, a NORAD spokesman said Monday.

The exercise was solely to test procedures and was no indication that NORAD had any reason to believe the scenario would happen in the real world, according to a spokesman.

It is unclear whether the simulated scenario was that of a hijacked plane being "used as missile" -- intentionally crashing into a building -- or that of an out-of-control hijacked plane.

Military officials said the exercise involved simulating a crash into a building that would be recognizable if identified, but was not the World Trade Center or the Pentagon.

They emphasize it involved an airliner being hijacked as it flew into U.S. airspace from abroad, a slightly different scenario from what happened on September 11, 2001.

The identity of the building named in the exercise is classified.

The exercise was conducted at one regional sector, and was not conducted at the headquarters, as are major exercises. This sector exercise involved some flying of military aircraft as well as a command post exercise in which communications procedures were practiced in an office environment.

NORAD officials emphasized that if it had been a real event, NORAD would have instituted standard procedures to try to contact the aircraft and keep it from crashing.

"We have planned and executed numerous scenarios over the years to include aircraft originating from foreign airports penetrating our sovereign airspace. Regrettably the tragic events of 9/11 were never anticipated or exercised," said Gen. Ralph Eberhart, commander of NORAD.

NORAD has the ongoing mission of defense of U.S. airspace.

According to a statement from NORAD, "Before September 11th, 01, NORAD regularly conducted a variety of exercises that included hijack scenarios. These exercises tested track detection and identification; scramble and interception; hijack procedures; internal and external agency coordination and operational security and communications security procedures."

All of those tasks are the responsibility of NORAD.

The statement continues:

"NORAD did not plan and execute these types of exercises because we thought the scenarios were probable. These exercises were artificial simulations that provided us the opportunity to test and validate our processes and rules of engagement with the appropriate coordination between NORAD's command headquarters, its subordinate regions and sectors and National Command authorities in Canada and the United States.

"Since 9/11 we have continued our exercise program having conducted more than 100 exercises, all of which have included mock hijacks. NORAD has flown 35,000 sorties and scrambled or diverted fighters from air patrols nearly 1,800 times. Additionally, NORAD fighters out of Florida have intercepted two hijacked aircraft since 9/11; both originating from Cuba and escorted to Key West in spring 2003. NORAD remains vigilant and its tolerance for any anomaly in the sky remains very low. The 9/11 commission has been informed about our exercises that include hijack scenarios.

"At the NORAD headquarters' level we normally conducted four major exercises a year, most of which included a hijack scenario. Since 9/11 however we have conducted more than 100 exercises, all of which included at least one hijack scenario."