"I wholeheartedly concurred in the decision he made, that if the plane would not divert, 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," Cheney said.
Mr. Bush gave the order after two planes slammed into the two towers in New York and a third rammed the Pentagon. A fourth plane also was hijacked and appeared to be on a course for Washington; that plane later crashed outside Pittsburgh after passengers apparently struggled with the hijackers.
Mr. Bush acknowledged later that he gave the order.
"I gave our military the orders necessary to protect Americans, do whatever it would take to protect Americans," Mr. Bush said, when asked whether he struggled with the decision.
"Never in anybody's thought process about how to protect Americans did we ever think the evil-doers would fly not one, but four commercial aircraft into precious U.S. targets. Never."
It was the toughest decision on a nightmarish day, Cheney told NBC's "Meet the Press."
"The toughest decision was this decision of whether or not we would intercept commercial aircraft on the day of the attacks," he said.
"People say that's a horrendous decision to make. Well, it is. You've got an airplane full of American citizens civilians captured by terrorists. Are you going to in fact shoot it down, obviously, and kill all those Americans on board?"
The alternative, he said, could have been worse.
"If we ... had the opportunity to take out the two aircraft that hit the World Trade Center, would we have been justified in doing it? I think absolutely we would have. "
Cheney's interview was his first appearance since being whisked away Thursday to the presidential retreat of Camp David, Maryland, to keep him at a safe distance from the president.
The two were reunited at Camp David for weekend government meetings on the crisis, ignited last Tuesday when hijackers slammed jetliners into the World Trade Center and Pentagon.
Cheney said he was physically picked up and rushed by Secret Service agents to an underground shelter at the White House in response to fears of an airliner attack on the building.
"They came in and said, 'Sir we have to leave immediately' and grabbed me. Your feet touch the floor periodically, but they're bigger than I am and they hoisted me up and moved very rapidly down the hallway," he said.
He then urged Mr. Bush not to return to Washington from Florida immediately. "I said, 'Delay your return. We don't know what's going on here, but it looks like we've been targeted,'" Cheney said.
He said he discussed with Mr. Bush what the president should say in his first statement dscribing the attacks as apparent terrorism, and kept in frequent contact with the president.
Cheney said he is convinced a telephone threat against Air Force One was credible, but acknowledged it "may have been phoned in by a crank."
"But in the midst of what was going on there was no way to know that," he said.
Mr. Bush has been criticized for delays in returning to Washington and for the White House handling seen by some as overly political of its belated disclosure of the threat against Air Force One. The White House has said the caller used code words that raised the credibility of the threat.
Cheney said he also ordered the evacuation of Cabinet members and was involved in the evacuation of House Speaker Dennis Hastert, second in line for the presidency after Cheney.
Cheney said he rejected Secret Service suggestions that he leave the White House.
After leaving the emergency shelter to which he was first taken, Cheney said, he worked out of "the Presidential Emergency Operations Center." He and top officials kept in contact with Air Force One, the Pentagon, CIA and other agencies, including over video links.
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