SMITHSBURG - Fearing the alternative, President Bush made a gut-wrenching decision Tuesday and ordered U.S. fighter pilots to shoot down a hijacked commercial airliner if the pilot refused orders to change course, which appeared to be set for Washington, D.C.

Bush, who was in Florida at the time, gave the order after speaking by phone with Vice President Dick Cheney. The vice president reflected on the decision yesterday, as he offered a candid account of his conversations with the president that morning, and of the tense scene inside the White House as the administration weighed its options, with the nation under attack.

"Now, people say, you know, that's a horrendous decision to make," Cheney said. "Well, it is. You've got an airplane full of American citizens, civilians, captured by hostages, captured by terrorists, ... and are you going to, in fact, shoot it down, obviously, and kill all those Americans on board?"

"You have to ask yourself," said the vice president, "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 doing that? I think absolutely we would have."

Cheney was speaking in a television interview with NBC at the Camp David presidential retreat, in the hills overlooking this Maryland town. Later yesterday, the president said that he never imagined being forced to make such a choice. His order was never carried out. The only remaining hijacked plane in the air - an aircraft that officials say was likely bound for Washington - went down in rural Somerset County, Pa.

"Obviously, when I was told what was taking place, when I was informed that an unidentified aircraft was headed to the heart of the capital, I was concerned," Bush told reporters after returning to the White House from Camp David.

"I wasn't concerned about my decision. I was more concerned about the lives of innocent Americans. I had realized there on the ground in Florida we were under attack. But never did I dream we would have been under attack this way."

Like many Americans, Cheney has freeze-framed exactly where he was as it became clear that the nation was being assaulted from the air. In Bush's absence, Cheney would make the crucial decisions at the White House. He ordered Cabinet officials and members of Congress to secure locations. He told the president that he should not return to Washington immediately from Florida. And he strongly urged Bush to order the fighter planes to fire at unauthorized airliners over Washington.

Just before 9 a.m. Tuesday, the vice president was convening a meeting with his speechwriter in his West Wing office. His secretary interrupted to tell him that a plane had just hit the World Trade Center in Manhattan.

"We turned on the television and watched for a few minutes, and then actually saw the second plane hit," Cheney said. "And as soon as that second plane showed up, that's what triggered the thought: terrorism, that this was an attack."

Cheney immediately ordered that the president's chief of staff, Andrew Card, be contacted in Florida. Card told Bush about the second attack at the World Trade Center - he already knew of the first - while he was speaking with elementary school children in Sarasota. After speaking with Cheney, Bush delivered a statement in the library at the Florida school, telling the nation that there was "an apparent terrorist attack" in New York City.

Meanwhile, the vice president called Condoleezza Rice, the national security adviser; Mary Matalin, his top aide; and other advisers to his office. The group was huddled in Cheney's office, watching the television for updates and discussing options, when Secret Service agents interrupted.

"Under these circumstances, they just move," recalled Cheney. "They don't say 'sir' or ask politely. They came in 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."

"They hoisted me up and moved me very rapidly ... into an underground facility under the White House. And, as a matter of fact, it's a corridor, locked at both ends, and they did that because they had received a report that an airplane was headed for the White House."

The plane was American Airlines Flight 77, which left Washington Dulles International Airport, flew west toward Ohio, was captured by hijackers and turned back sharply.

"As best we can tell, they came initially at the White House," said Cheney. "And when it entered the danger zone and looked like it was headed for the White House was when they grabbed me and evacuated me to the basement. The plane obviously didn't hit the White House. It turned away and, we think, flew a circle and came back in and then hit the Pentagon."

Elsewhere in the West Wing, the Secret Service was evacuating staff, as well as reporters, who were at their desks near the White House briefing room. The agents had to nearly force one reporter out of the building after he refused to leave, saying he had to stay at his computer and send out bulletins about what was transpiring.

From the subterranean shelter, the vice president got on the phone and called Bush in Florida again and told him not to 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,'" the vice president said.

The president would then fly to Louisiana and then Nebraska, returning to Washington at dusk. The decision to keep Bush away from Washington for so many hours sparked some criticism. Cheney defended the decision yesterday. "The most important thing here is to preserve the presidency," he said.

Cheney said they didn't know how long the attack would last, or how many more planes were on the way. "Within about 35 or 40 minutes, we'd seen this unfolding of this monstrous terrorist attack," he said. "It was absolutely the right decision."