The extraordinary decision, which the president described as "difficult," was first disclosed earlier in the day by Vice President Dick Cheney and was later confirmed by Bush.
The presidential order was never acted upon. It was made after two jetliners already had sliced into the World Trade Center and a third had slammed into the Pentagon, officials said.
Twenty minutes after the Pentagon was struck, a fourth hijacked jetliner crashed in rural Pennsylvania, about 25 minutes by air from Washington. That crash was believed to have followed an attempt by passengers to rush the hijackers and retake the aircraft.
Had United Airlines Flight 93, originally bound from Newark, N.J., to San Francisco, continued on its deviated course toward Washington, it could have been intercepted by jet fighters, U.S. defense officials have said.
Two F-16 fighters were scrambled from Langley Air Force Base in Virginia 20 minutes before the Boeing 757 went down. Bush said he made his decision after being "informed that an unidentified aircraft was headed to the heart of the capital," apparently a reference to Flight 93.
"I wasn't concerned about my decision," the president said. "I was more concerned about the lives of innocent Americans."
Cheney, in an interview Sunday morning on NBC's "Meet the Press," said he had "wholeheartedly concurred" with Bush's decision. At the time, he said, it was feared that as many as six aircraft might have been hijacked.
The vice president described the order this way: "If the plane would not divert, or if they wouldn't pay any attention to instructions to move away from the city, as a last resort our pilots were authorized to take them out.
"Now people say, you know, that's a horrendous decision to make. Well, it is. You've got an airplane full of American citizens, civilians, captured by . . . terrorists and you are going to, in fact, shoot it down . . . and kill all those Americans on board.
". . . You have to ask yourself: If we had had combat air patrol up over New York, and we'd had the opportunity to take out the two aircraft that hit the World Trade Center, would we have been justified in it? And I think absolutely we would have."
Elaborate Procedures to Handle Hijackings
Asked if the response ordered by the president would become policy in any future terrorist hijackings, Cheney said: "It's a presidential-level decision."
The military has elaborate procedures for how pilots should deal with hijacked airliners--contacting the commandeered aircraft, trying to divert it and, as a last resort, seeking presidential approval to open fire.
These procedures had rarely, if ever, been practiced. Before Tuesday, the possibility that U.S. military planes might be forced to shoot down commercial airliners to prevent terrorist suicide attacks had seemed beyond remote.
Said Bush, who as a young man flew fighter jets for the Texas Air National Guard: "Never in anybody's thought process about how to protect America did we ever think that the evildoers would fly not one but four commercial aircraft into precious U.S. targets. Never."
"Do we train our pilots to shoot down commercial airliners filled with American civilians? No," Cheney said. "That's not a mission they've ever been given before. Now we've got to think about that."
Maj. Gen. Paul A. Weaver, director of the Air National Guard, said that since Tuesday, commanders and pilots had been discussing the ramifications of the terrorists' use of commercial airliners as guided missiles.
"This is new territory for all of us," he said. "Air crew members are going through a lot of soul-searching."
Two sets of military jets were ordered aloft Tuesday morning, but in each instance they were unable to intercept hijacked airliners before they crashed.
At 8:44 a.m. Tuesday, after it became clear that American Airlines Flight 11 from Boston to Los Angeles had been hijacked and had turned south toward New York City, two F-15 jet fighters were scrambled from Otis Air Force Base on Cape Cod, Mass.
Fighter Jets Unable to Reach Targets
The airliner rammed into the trade center's north tower, however, before the fighters managed to take off. They had been flying only nine minutes when a second hijacked flight, United Airlines Flight 175 from Boston to Los Angeles, struck the trade center's south tower.
The two F-16s that scrambled from Langley had been in the air only two minutes when American Flight 77 crashed into the Pentagon. It was not clear if these fighters stayed aloft to seek out the fourth hijacked jetliner.
In his television appearance, Cheney said: "We did, in effect, put a flying combat air patrol up over the city, F-16s with an AWACS, which is an airborne radar system, and tanker support so they could stay up a long time.
"It doesn't do any good to put up a combat air patrol if you don't give them instructions to act if, in fact, they feel it's appropriate."
Cheney described in detail how the White House reacted in the first confusing hours Tuesday. When the first plane struck the trade center, Bush was in Florida, his motorcade nearing its destination, the Emma E. Booker Elementary School in Sarasota.
Cheney was watching television coverage of the first crash when the second plane struck. Immediately, he said, that "triggered the thought of terrorism" and he summoned Bush's national security advisor, Condoleezza Rice, and called Bush.
From a hurried phone conversation with Cheney came the statement Bush made minutes later describing the attacks as an act of apparent terrorism. Moments later, the president was on his way to the Sarasota-Bradenton airport and Cheney was whisked out of his office by Secret Service agents.
"They came in," he recalled, "and said, 'Sir, we have to leave immediately,' and grabbed me . . . and, you know, your feet touch the floor periodically, but they're bigger than I am and they, of course, they have to move me very rapidly down the hallway, down some stairs, through some doors, and down some more stairs into an underground facility under the White House, what's in effect a corridor locked at both ends. And they did that because they'd received a report that an airplane was headed for the White House."
This would have been American Flight 77, which flew west from Dulles International Airport outside the capital. It had been commandeered somewhere over Pennsylvania or Ohio and turned back toward Washington.
"As best we can tell, they came initially at the White House," Cheney said, "didn't circle it, but was headed on a track into it. . . . And when it entered the danger zone and looked like it was headed to the White House is when they grabbed me and evacuated me to the basement."
The plane "turned away and, we think, flew a circle and came back in and then hit the Pentagon," he said.
The vice president said he suspected the Boeing 757 did not attack the White House because "it turned out to be tougher to see than they had anticipated." Coming in low from the west, he said, the terrorists' view of the White House might have been blocked by the large Old Executive Office Building.
"And I'm speculating that the lack of ability to be able to acquire the visual aid may, in fact, have led them to go back" toward the Pentagon.
Once in the shelter, Cheney said, he reached for a secure telephone and again called the president and strongly urged him "to delay his return" to Washington.
"I said, 'Delay your return. We don't know what's going on here, but it looks like, you know, we've been targeted,' " Cheney said.
He said initial reports from federal aviation officials indicated that as many as six airplanes might have been hijacked. Terrorists had turned off transponder tracking devices in at least two of the aircraft, creating confusion over the precise number.
"That's what we started working off of," the vice president said, "that list of six, and we could account for two of them in New York. The third one we didn't know what had happened to. It turned out it had hit the Pentagon, but the first reports on the Pentagon attack suggested a helicopter and then later a private jet."
After eyewitnesses confirmed a commercial airliner in fact had crashed into the Pentagon, Cheney and other White House officials trying to piece together what was happening from their bunker still believed three hijacked aircraft were at large.
Cheney said: "We would have been absolute fools not to go into button-down mode, make sure we had successors evacuated, make sure the president was safe and secure."