NEW YORK -- Mounting an audacious attack against the United States, terrorists crashed two hijacked airliners into the World Trade Center and brought down the twin 110-story towers Tuesday morning. A jetliner also slammed into the Pentagon as the government itself came under attack.

	Horrified onlookers watch the World Trade Center burn in New York.

Thousands could be dead or injured, a high-ranking New York City police official said, speaking on condition of anonymity.

A fourth jetliner, also apparently hijacked, crashed in Pennsylvania.

President Bush ordered a full-scale investigation to "hunt down the folks who committed this act."

Authorities had been trying to evacuate those who work in the twin towers when the glass-and-steel skyscrapers came down in a thunderous roar within about 90 minutes after the attacks, which took place minutes apart around 9 a.m. But many people were thought to have been trapped. About 50,000 people work at the Trade Center and tens of thousands of others visit each day.

American Airlines said two of its planes, both hijacked, crashed with a total of 156 people aboard, but said it could not confirm where they went down. Two United airliners with a total of 110 aboard also crashed -- one outside Pittsburgh, the other in a location not immediately identified. Altogether, the planes had 266 people aboard.

"This is perhaps the most audacious terrorist attack that's ever taken place in the world," said Chris Yates, an aviation expert at Jane's Transport in London. "It takes a logistics operation from the terror group involved that is second to none. Only a very small handful of terror groups is on that list. ... I would name at the top of the list Osama bin Laden."

Within the hour, the Pentagon took a direct, devastating hit from a plane. The fiery crash collapsed one side of the five-sided structure.

The White House, the Pentagon and the Capitol were evacuated along with other federal buildings in Washington and New York.

The president put the military on its highest level of alert. Authorities in Washington immediately called out troops, including an infantry regiment, and the Navy sent aircraft carriers and guided missile destroyers to New York and Washington. The U.S. and Canadian borders were sealed, security was tightened at naval installations and other strategic points, and all commercial air traffic across the country was halted until at least noon on Wednesday.

"This is the second Pearl Harbor. I don't think that I overstate it," said Sen. Chuck Hagel, R-Neb. The Dec. 7, 1941, Japanese attack on Pearl Harbor killed nearly 2,400 people and drew the United States into World War II.

Sen. John McCain, R-Ariz., said: "These attacks clearly constitute an act of war."

In June, a U.S. judge had set this Wednesday as the sentencing date for a bin Laden associate for his role in the 1998 bombing of a U.S. embassy in Tanzania that killed 213 people. The sentencing had been set for the federal courthouse near the World Trade Center. No one from the U.S. attorney's office could be reached Tuesday to comment on whether the sentencing was still on.

Afghanistan's hardline Taliban rulers condemned the attacks and rejected suggestions that bin Laden was behind them, saying he does not have the means to carry out such well-orchestrated attacks. Bin Laden has been given asylum in Afghanistan.

Abdel-Bari Atwan, editor of the Al-Quds al-Arabi newspaper, said he received a warning from Islamic fundamentalists close to bin Laden, but did not take the threat seriously. "They said it would be a huge and unprecedented attack but they did not specify," Atwan said in a telephone interview in London.

In the West Bank city of Nablus, thousands of Palestinians celebrated the attacks, chanting "God is Great" and handing out candy.

In New York, Mayor Rudolph Giuliani said about 600 of the injured were taken to area hospitals, 150 of them in critical condition. It could take weeks to dig through the rubble for victims.

American Airlines initially identified the planes that crashed into the Trade Center as Flight 11, a Los Angeles-bound jet hijacked after takeoff from Boston with 92 people aboard, and Flight 77, which was seized while carrying 64 people from Washington to Los Angeles.

Law enforcement officials, speaking on condition of anonymity, said it was Flight 77 that hit the Pentagon.

In Pennsylvania, United Airlines Flight 93, a Boeing 757 en route from Newark, N.J., to San Francisco, crashed about 80 miles southeast of Pittsburgh with 45 people aboard. United said another of its planes, Flight 175, a Boeing 767 bound from Boston to Los Angeles with 65 people on board, also crashed, but it did not say where. The fate of those aboard the two planes was not immediately known.

United's pilots union said United Flight 175 crashed into the Trade Center. But the airline had no immediate comment.

An emergency dispatcher in Westmoreland County, Pa., received a cell phone call at 9:58 a.m. from a man who said he was a passenger locked in the bathroom of United Flight 93, said dispatch supervisor Glenn Cramer.

"We are being hijacked, we are being hijacked!" Cramer quoted the man as saying. The man told dispatchers the plane "was going down. He heard some sort of explosion and saw white smoke coming from the plane and we lost contact with him," Cramer said.

Evacuations were ordered at the United Nations in New York and at the Sears Tower in Chicago. Los Angeles mobilized its anti-terrorism division. Walt Disney World in Orlando, Fla., was evacuated, and Hoover Dam on the Arizona-Nevada line was closed to visitors.

In New York, hours after the attacks, huge clouds of smoke billowed from the ruins, obscuring much of the skyline.

The two planes blasted fiery, gaping holes in the upper floors of one of New York's most famous landmarks and rained debris on the streets. About an hour later, the southern tower collapsed with a roar and a huge cloud of smoke; the other tower fell about a half-hour after that, covering lower Manhattan in heaps of gray rubble and broken glass. Firefighters trapped in the rubble radioed for help.

"All this stuff started falling and all this smoke was coming through. People were screaming, falling, and jumping out of the windows," from high in the sky, said Jennifer Brickhouse, 34, of Union, N.J., who was going up the escalator into the World Trade Center.

On the street, a crowd mobbed a man at a pay phone, screaming at him to get off the phone so that they could call relatives. Dust and dirt flew everywhere. Ash was 2 to 3 inches deep in places. People wandered dazed and terrified.

"I have a sense it's a horrendous number of lives lost," Mayor Rudolph Giuliani said. "Right now we have to focus on saving as many lives as possible."

The death toll on the crashed planes alone could surpass that of the Oklahoma City bombing on April 19, 1995, which claimed 168 lives in what was the deadliest act of terrorism on U.S. soil.

"Today we've had a national tragedy," Bush said in Sarasota, Fla. "Two airplanes have crashed into the World Trade Center in an apparent terrorist attack on our country." He said he would be returning immediately to Washington.

John Axisa, who was getting off a commuter train to the World Trade Center, said he saw "bodies falling out" of the building. He said he ran outside, and watched people jump out of the first building. Then there was a second explosion, and he felt heat on the back of neck.

People ran down the stairs in panic and fled the building. Thousands of pieces of what appeared to be office paper drifted over Brooklyn, about three miles away.

Several subway lines were immediately shut down. Trading on Wall Street was suspended. New York's mayoral primary election Tuesday was postponed. All bridges and tunnels into Manhattan were closed.

David Reck was handing out literature for a candidate for public advocate a few blocks away when he saw a jet come in "very low, and then it made a slight twist and dove into the building."

Terrorist bombers struck the World Trade Center in February 1993, killing six people and injuring more than 1,000 others.

"It's just sick. It just shows how vulnerable we really are," Keith Meyers, 39, said in Columbus, Ohio. "It kind of makes you want to go home and spend time with your family. It puts everything in perspective," Meyers said. He said he called to check in with his wife. They have two young children.

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.

In Florida, Bush was reading to children in a classroom at 9:05 a.m. when his chief of staff, Andrew Card, whispered into his ear. The president briefly turned somber before he resumed reading. He addressed the tragedy about a half-hour later.