WASHINGTON (AP) - The Pentagon took a direct, devastating hit from an aircraft and the enduring symbols of American power were evacuated Tuesday as an apparent terrorist attack quickly spread fear and chaos in the nation`s capital. President Bush ordered the nation`s military to ``high-alert status,`` and vowed to ``hunt down and punish those responsible`` for the attacks in Washington and New York, where the World Trade Center was devastated with a heavy loss of life. The president was in Florida at the time of the attacks, and was flown at midday to the safety of a military installation, Barksdale Air Force Base in Louisiana. The top leaders of Congress were led to the safety of an undisclosed location, and military aircraft were reported patrolling the skies above the capital. The nerve center of the nation`s military burst into flames and a portion of one side of the five-sided structure collapsed when the plane struck in midmorning. Secondary explosions were reported in the aftermath of the attack and great billows of smoke drifted skyward toward the Potomac River and the city beyond. At midday, local hospitals reported receiving 40 victims of the attack, with seven patients in critical condition admitted to one facility for treatment of burns. ``The whole building shook`` with the impact, said Terry Yonkers, an Air Force civilian employee at work inside the Pentagon at the time of the attack. ``There was screaming and pandemonium,`` he said, but the evacuation ordered shortly afterward was carried out smoothly. ``I saw a big jet flying close to the building coming at full speed. There was a big noise when it hit the building,`` said Oscar Martinez, who witnessed the attack. Authorities have not described the plane that hit the Pentagon, although eyewitnesses said it was clearly a big commercial jet. Vice President Dick Cheney was in Washington and he and first lady Laura Bush were taken to an undisclosed secure location, officials said. Congressional leaders were hustled away from the Capitol to safety. ``The leadership of the Defense Department is OK. The secretary (Donald H. Rumsfeld) is OK,`` Pentagon spokesman Glenn Flood told reporters. Authorities immediately began deploying troops, including a regiment of light infantry, in response to an attack for which they said there had been no advance warning. The departments of Justice, State, Treasury and Defense and the Central Intelligence Agency were evacuated - an estimated 20,000 at the Pentagon alone. Agents with automatic weapons patrolled the White House grounds. And the FAA ordered the entire nationwide air traffic system shut down for the first time in history. Officials said two planes had crashed into the World Trade Center, and a third into the Pentagon. A fourth plane crashed 80 miles southeast of Pittsburgh, and one lawmaker, Rep. James Moran, D-Va., said after a Marine Corps briefing that ``it was apparently intended to Camp David,`` the presidential retreat in the mountains of Maryland. Armed personnel secured the runway at Barksdale Air Force Base when Bush`s plane touched down there. After a short stay on the ground, Bush`s plane prepared to take off once again, to an undisclosed destination. Bush and others spoke freely about terrorism being the cause, and already there was speculation about those responsible. One U.S. official, speaking on condition of anonymity, said there were unspecified indications that Osama bin Laden`s organization was responsible. But finally assessing responsibility was likely months if not years down the road, and in the meantime, there was no attempt to minimize the impact. The military was ordered to ``Threat Level Delta,`` the highest level, at least in the Washington area, according to Air Force Capt. Tatiana Stead at Andrews Air Force Base. ``This is the second Pearl Harbor. I don`t think that I overstate it,`` said Sen. Chuck Hagel, R-Neb., referring to the attack 60 years ago that surprised the nation`s intelligence apparatus and propelled the country into World War II. With Bush away from the capital, his advisers were preparing a list of options, including closing the nation`s borders, according to a senior U.S. official. The source, who spoke on condition of anonymity, said it was premature to discuss military options because investigators were still trying to determine who was responsible for the attacks. Away from the Pentagon, unexplained explosions were reported in the vicinity of the State Department and the Capitol. A torrent of people rushed from their office buildings throughout the nation`s capital, eager to leave a city under siege. The cell phone networks were overloaded, clusters of people sprayed on the sidewalks and at least one suburban school district announced plans to close early. The Pentagon was hit a short while after the World Trade Center was struck. a plane, described by witnesses as a jetliner, made impact in the portion of the building on side opposite from where Rumsfeld`s office are located. Paul Begala, a Democratic consultant, said he witnessed an explosion near the Pentagon, saying it sent a huge, orange fireball skyward. AP reporter Dave Winslow also saw the crash. He said, ``I saw the tail of a large airliner. ... It plowed right into the Pentagon.`` Gen. Richard Myers, vice chairman of the Joint Chiefs of Staff, said that prior to the crash into the Pentagon, military officials had been notified that another hijacked plane had been heading from the New York area to Washington. He said he assumed that hijacked plane was the one that hit the Pentagon, though he couldn`t be sure. Asked if there was any possibility the crashes were anything other than deliberate, a government official said it appeared not to be an accident.