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At Barksdale Air Force Base in northwest Louisiana, President Bush told reporters: "Freedom itself was attacked this morning by a faceless coward, and I assure you freedom will be defended," Bush said. "Make no mistake. The United States will hunt down and pursue those responsible for these cowardly actions." The U.S. military is on its highest state of alert.

The president had started the day at an elementary school in Florida, but landed in Louisiana shortly before noon. He was to leave there on Air Force One and continue traveling to undisclosed locations, said ABCNEWS White House correspondent Ann Compton, who was with the president. The pared-down press corps traveling with the president were not being told where they were going.

In the wake of the attacks, the U.S. Capitol, White House and other federal buildings were evacuated in Washington. In New York, U.N. headquarters was also evacuated, as were skyscrapers in several other cities.

There were no immediate details available on casualties, but thousands of people work in the buildings affected in both New York and Washington.

Chronology of Destruction

The chaos began at about 8:50 a.m. ET when a hijacked American Airlines passenger plane smashed into the One World Trade Center, the northern tower. Then, at 9:04 a.m., another jet crashed into the southern tower, Two World Trade Center.

According to the FBI, the planes involved in the trade center crashes were American Airlines Flight 11, a Boeing 767 en route from Boston to Los Angeles carrying 81 passengers, nine flight attendants and two pilots, and United Airlines Flight 175, a Boeing 767 also headed from Boston to Los Angeles. Officials said the plane was carrying 56 passengers, two pilots and seven flight attendants.

At about 10 a.m., the southern tower collapsed, enveloping lower Manhattan in a cloud of dust, ash and debris. A half-hour later, the northern tower also fell in on itself.

"Lots of smoke and then the next thing I heard was an explosion in the building from the top, the south building just crumbled, just completely went down, I saw it," said witness Joan Fleischer. "It's hard to see all the pieces, but you could see it tipping over and just crashing to the ground."

Two planes crashed into the World Trade Center. (ABCNEWS.com/ Magellan Geographix)

Part of the Pentagon also collapsed after a plane crashed into it shortly after the disasters in New York. The building was on fire, and military personnel were evacuated.

Shortly after the World Trade Center catastrophe, at about 9:38 a.m., FBI officials say American Airlines Flight 77, a Boeing 757 en route from Washington Dulles Airport to Los Angeles crashed into the Pentagon. The plane was carrying 58 passengers, four flight attendants and two pilots.

Then, at about 10:20 a.m., United Airlines Flight 93, a Boeing 757, en route from Newark, N.J., to San Francisco, crashed in western Pennsylvania in Somerset County about 80 miles southeast of Pittsburgh. The plane was carrying 38 passengers, two pilots and five flight attendants.

Highest Level of Alert

All military bases were put on Threatcon Alpha, the highest level of alert. The FAA shut down all air traffic in the country at 9:25 a.m. ET, ordering any planes in the air to land at the nearest airport. It was the first time the FAA ever ordered a nationwide "groundstop."

All trans-Atlantic flights were re-routed to airports in Canada.

In response to today's attacks, the U.S. Atlantic Fleet based in Norfolk, Va., has sent ships to sea to contribute to the "air defense" of Washington, D.C. and New York City. Other ships have been directed to get underway with embarked personnel and medical assets to provide humanitarian and medical support.

At least one of the planes involved in today's crashes was able to communicate that it had been hijacked, FBI officials said.

Law enforcement officials at the highest levels are stunned at the level and sophistication of the attacks, officials said, and have launched a massive probe. Although it is still too early to know the source of the attacks, the earliest theories are focusing on a connection to the turmoil in the Middle East.

Suspected terrorist Osama bin Laden warned three weeks ago that his followers would carry out an "unprecedented attack" on the United States, an Arab journalist told Reuters news agency.

"It is most likely the work of Islamic fundamentalists. Osama bin Laden warned three weeks ago that he would attack American interests in an unprecedented attack, a very big one," said Abdel-Bari Atwan, editor of the London-based al-Quds al-Arabi.

"Personally we received information that he planned very, very big attacks against American interests," he said. "We received several warnings like this. We did not take it so seriously, preferring to see what would happen before reporting it."

At least one military fighter jet patrolled the skies above Manhattan this morning in the wake of the tragedy.

Protecting the White House

As the morning went on, and the horror of the initial crash grew with each new disaster, officials in Washington and New York moved to try to limit any further loss of life.

Police in Washington closed off a two-block perimeter around the White House and agents with automatic weapons and machine guns moved tourists out of the area, locking down entire buildings around the area.

Cell phones may have been deliberately turned off by servers because of fear the phones could be used to detonate a bomb.

In New York, Mayor Rudolph Giuliani ordered that lower Manhattan be evacuated, asking everyone below Canal Street to walk north out of the area. The city's subway system was also shut down.

"I would like to take this opportunity to tell everyone to remain calm and to the extent that they can, evacuate lower Manhattan," Giuliani said in a broadcast on local cable news channel NY1.

An evacuation was ordered at the Sears Tower in Chicago, Los Angeles mobilized its anti-terrorism division, the Space Needle in Seattle was closed, and security was intensified around the naval facilities in Hampton Roads, Va. The financial markets in New York were closed.

‘Unbelievable, Unbelievable, Unbelievable’

Palestinian leader Yasser Arafat condemned the aircraft attacks, shortly after an anonymous caller told Abu Dhabi television that a radical Palestinian group was responsible for the attacks. The group later denied any involvement.

A plane crashed into the Pentagon. (ABCNEWS.com/ Magellan Geographix)

"I send my condolences, the condolences of the Palestinian people to American President Bush and his government and to the American people for this terrible act," Arafat told reporters in Gaza. "We completely condemn this serious operation … We were completely shocked. It's unbelievable, unbelievable, unbelievable."

In lower Manhattan, as police, fire and emergency personnel tried to cope with the mayhem, witnesses were in shock.

"I'm sitting down and I'm crying and I couldn't believe that something like this could actually happen," said Tony Bristow, who was working on a nearby pier when the planes smashed in the towers. "Then about 10 minutes later the whole building just started to collapse and now two seconds ago both of them collapsed and now there's no more World Trade Center. It's — this is ridiculous. I don't believe this." Before they collapsed, large holes were visible in sides of the 110-story buildings, landmark twin towers, which were struck by terrorist bombers in February 1993.

The tops of the twin towers were obscured by the smoke, and thousands of pieces of what appeared to be office paper came drifting over Brooklyn, about 3 miles from the tower.

The World Trade Center bombing on Feb. 26, 1993, left six people dead and injured more than 1,000 others.

In 1945, an Army Air Corps B-25, a twin-engine bomber, crashed into the 79th floor of the Empire State Building in dense fog.

ABCNEWS' Geraldine Sealey, Dean Schabner, Michael McAuliff and Corey Goldman contributed to this report.