Confusion, uncertainty, poor communication and a general lack of preparedness kept Defense Department leaders from evacuating the Pentagon during the 35 minutes between the time the second of two commercial airliners struck New York's World Trade Center towers and the time a third terrorist-driven plane slammed into the Pentagon, according to defense officials.

The nation's military air defense command received word from the Federal Aviation Administration that a hijacked commercial airliner was heading toward Washington 12 minutes before it hit, according to a chronology prepared by the Pentagon. But until the moment of impact, Defense Secretary Donald H. Rumsfeld and his top aides were unaware of any imminent danger. And Pentagon authorities responsible for guarding the building, also given no warning, took no steps to order an evacuation or otherwise alert the building's 20,000 employees.

Two F-16 fighter jets were scrambled immediately in response to the FAA notice, but they did not take off from Andrews Air Force Force Base. Andrews is only 15 miles from the Pentagon, but it had no planes on alert for continental air defense. Instead, the jets departed from Virginia's Langley Air Force Base, about 130 miles from the Pentagon. Two minutes after they made it into the air -- and with another 20 minutes to go before reaching the skies over Washington -- American Airlines Flight 77 crashed into the Pentagon, killing 188 people, including everyone on the plane.

As new details emerged yesterday about the military's initial reactions to Tuesday's terrorist attacks, they revealed just how ill-prepared the Pentagon was -- not only to protect itself but also to guard U.S. skies and American lives against commercial airliners turned into deadly weapons.

Three Air Force generals, all sharing some responsibility for America's air defense, described in interviews how inadequate the procedures were for dealing with what was occurring. "This was something we had never seen before, something we had never even thought of," one said.

The officers said that even if U.S. military interceptors had arrived over New York or Washington ahead of the hijacked planes, Air Force pilots and air controllers had never practiced procedures for confronting hijacked commercial airliners on suicide missions over urban centers. Their exercises had always involved intercepts outside U.S. borders, over the Atlantic and Pacific, with time to attempt to divert the aircraft, interrogate its crew and consult with the White House before undertaking more drastic measures. Under standing rules of engagement, a decision to shoot down a commercial aircraft generally would require presidential approval, Air Force officials said.

The assaults revealed a gaping hole in America's air defense system, which was designed during the Cold War to block Soviet bombers from entering American airspace, not to intercept commercial airliners that took off from domestic airports and were seized by terrorists.

But the lack of readiness also extended to operations on the ground, which were hampered by a slowness among senior Pentagon officials to grasp the full extent of the threat. While many in the Pentagon were immediately aware of the attack on the New York towers -- many even had their office televisions turned on in time to see the second plane strike -- there was little thought given to the possibility that the nation's military center was itself at risk.

Rumsfeld, informed by his chief of staff of the New York attacks, stayed in his office on the east side of the Pentagon for a scheduled CIA briefing. Several of his senior aides rushed to the Pentagon's command center deep within the five-sided complex, where a crisis action center was being set up. But they remained unaware that another hijacked plane was headed in their direction. And even after the plane hit, they did not immediately realize what caused the jolt that rumbled through the building and the smoke that started billowing from the building's west side.

"The first thought everyone had was that it had been a bomb," said Victoria Clarke, Rumsfeld's spokeswoman.

So unsuspecting were defense officials of the possibility of an attack that even after the assault on the New York towers, they took few measures to bolster security. Once the second tower was struck, John Jester, chief of the Defense Protective Service, which guards the Pentagon, ordered the building's threat level up just one notch, from "normal" to "alpha," which simply called for spot-inspections of vehicles and increased police patrols.

Because the Pentagon lies under some of the approach and departure routes used by Ronald Reagan National Airport, defense officials were accustomed to aircraft frequently passing above. But the Pentagon had no anti-aircraft guns posted on its roof, nor any radars of its own for tracking local air traffic.

"There's nothing like that organic in the Pentagon," a four-star Army general said.

While security officials had run drills for what would happen if a plane crashed into the building, the scenarios all had assumed such a crash would be accidental and involve a small, propeller-driven commuter aircraft -- the kind that tends to pass over the Pentagon -- not a larger, jet-powered plane like American Airlines Flight 77, a Boeing 757.

"A terrorist plane is obviously just a very difficult threat to deal with, especially in an urban environment," Jester said.

As for evacuating the Pentagon, Jester said practice drills had been run about once every three months, although each would involve emptying only a section of the building. About a month ago, Jester said, a small Pentagon fire that generated lots of smoke resulted in a larger evacuation, but even then, only half the giant building was affected.

But Jester and other officials noted that Tuesday's damage and loss of life could have been considerably worse had it not been for some recent security improvements. A Pentagon news conference yesterday called attention to the fact that the section of the building that was hit had just been renovated under a planned 20-year rebuilding program. According to Lee Evey, the renovation program manager, a web of new steel reinforcements delayed the structure from caving in for about 30 minutes, long enough for hundreds of employees to escape. Some offices also were shielded by 2-inch-thick, blast-resistant windows and walls lined with bullet-proof kevlar cloth. New automatic fire doors and fire sprinklers also were functioning.

During the early years of the Cold War, the Pentagon kept thousands of planes on alert around the country for air defense, ready to fly against Soviet bombers within minutes. But over the past decade, the number dwindled to about 20 planes spread among seven bases. Days before last week's attacks, Air Force planners had pressed the service's leadership to make further cuts in continental air defenses to free up money for investment in future technologies, according to one general officer.

Responsibility for continental air defense has rested since 1958 with the North American Aerospace Defense Command (NORAD), whose operations center is encased deep inside a Colorado mountain to improve its chances of withstanding a nuclear attack. Crews there still monitor radar screens around the clock, looking for signs of a missile attack or suspicious aircraft heading for the United States.

But NORAD's mission has always been directed at monitoring planes originating from abroad, rather than domestic flights. NORAD does maintain close communications with the FAA, and civilian air traffic controllers have procedures for alerting the military of suspicious aircraft activity.

Since Tuesday's tragedy, the Defense Department has raised the number of bases with planes on alert to 26. It has also announced plans to call up thousands of Air Force reservists, suggesting the additional aircraft will remain on alert for months to come.