One of the imagined targets was the World Trade Center. In another exercise, jets performed a mock shootdown over the Atlantic Ocean of a jet supposedly laden with chemical poisons headed toward a target in the United States. In a third scenario, the target was the Pentagon — but that drill was not run after Defense officials said it was unrealistic, NORAD and Defense officials say.

NORAD, in a written statement, confirmed that such hijacking exercises occurred. It said the scenarios outlined were regional drills, not regularly scheduled continent-wide exercises.

 "Numerous types of civilian and military aircraft were used as mock hijacked aircraft," the statement said. "These exercises tested track detection and identification; scramble and interception; hijack procedures; internal and external agency coordination and operational security and communications security procedures."

A White House spokesman said Sunday that the Bush administration was not aware of the NORAD exercises. But the exercises using real aircraft show that at least one part of the government thought the possibility of such attacks, though unlikely, merited scrutiny.

On April 8, the commission investigating the Sept. 11 attacks heard testimony from national security adviser Condoleezza Rice that the White House didn't anticipate hijacked planes being used as weapons.

On April 12, a watchdog group, the Project on Government Oversight, released a copy of an e-mail written by a former NORAD official referring to the proposed exercise targeting the Pentagon. The e-mail said the simulation was not held because the Pentagon considered it "too unrealistic."

President Bush said at a news conference Tuesday, "Nobody in our government, at least, and I don't think the prior government, could envision flying airplanes into buildings on such a massive scale."

The exercises differed from the Sept. 11 attacks in one important respect: The planes in the simulation were coming from a foreign country.

Until Sept. 11, NORAD was expected to defend the United States and Canada from aircraft based elsewhere. After the attacks, that responsibility broadened to include flights that originated in the two countries.

But there were exceptions in the early drills, including one operation, planned in July 2001 and conducted later, that involved planes from airports in Utah and Washington state that were "hijacked." Those planes were escorted by U.S. and Canadian aircraft to airfields in British Columbia and Alaska.

NORAD officials have acknowledged that "scriptwriters" for the drills included the idea of hijacked aircraft being used as weapons.

"Threats of killing hostages or crashing were left to the scriptwriters to invoke creativity and broaden the required response," Maj. Gen. Craig McKinley, a NORAD official, told the 9/11 commission. No exercise matched the specific events of Sept. 11, NORAD said.

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

NORAD, a U.S.-Canadian command, was created in 1958 to guard against Soviet bombers.

Until Sept. 11, 2001, NORAD conducted four major exercises a year. Most included a hijack scenario, but not all of those involved planes as weapons. Since the attacks, NORAD has conducted more than 100 exercises, all with mock hijackings.

NORAD fighters based in Florida have intercepted two hijacked smaller aircraft since the Sept. 11 attacks. Both originated in Cuba and were escorted to Key West in spring 2003, NORAD said.