Associated Press

Ash covers a street in downtown New York City after the collapse of the World Trade Center following a terrorist attack Tuesday, Sept. 11, 2001.

Associated Press

A jet crashes into the World Trade Center in New York, Tuesday, Sept. 11, 2001. Terrorists crashed two planes into the World Trade Center, and the twin 110-story towers collapsed Tuesday morning.

Associated Press

A man coated with ash and debris from the collapse of the World Trade Center south tower coughs near City Hall in lower Manhattan Sept. 11, 2001.

Associated Press

People run from the collapse of World Trade Center Tower, Tuesday, Sept. 11, 2001 in New York. In a horrific sequence of destruction, terrorists hijacked two airliners and crashed them into the World Trade Center in a coordinated series of attacks that brought down the twin 110-story towers.

Associated Press

Pedestrians flee the area of the World Trade Center as the center's south tower crashes following a terrorist attack on the New York landmark Tuesday, Sept. 12, 2001.

Associated Press

The south tower of the World Trade Center begins to collapse following a terrorist attack on the New York landmark Tuesday, Sept. 11, 2001.

(AP Photo/Ernesto Mora)

This is the first in a series of three images showing the northern tower of the World Trade Center collapsing in New York, Tuesday, Sept. 11, 2001.

(AP Photo/Chao Soi Cheong)

Smoke, flames and debris erupts from one of the World Trade Center towers as a plane strikes it Tuesday, September 11, 2001. The first tower was already burning following a terror attack minutes earlier. Terrorists crashed planes into the two buildings and collapsed both towers.

Associated Press

Smoke billows from one of the towers of the World Trade Center and flames and debris explode from the second tower, Tuesday, Sept. 11, 2001. In one of the most horrifying attacks ever against the United States, terrorists crashed two airliners into the World Trade Center in a deadly series of blows that brought down the twin 110-story towers.

Associated Press

Firefighters battle a blaze at the Pentagon Tuesday, Sept. 11, 2001, after the building took a direct, devastating hit from an aircraft.

Associated Press

Smoke billows from the Pentagon in Washington, Tuesday, Sept. 11, 2001 after the building took a direct, devastating hit from a aircraft. The enduring symbols of American power were evacuated as an apparent terrorist attack quickly spread fear and chaos in the nation's capital.

Tuesday September 11, 2001

NEW YORK (AP) In a horrific sequence of destruction, terrorists crashed two planes into the World Trade Center, and the twin 110-story towers collapsed Tuesday morning. An aircraft also crashed at the Pentagon in apparent coordinated series of attacks that spread fear across the nation.

"I have a sense it's a horrendous number of lives lost," Mayor Rudolph Giuliani said. "I don't know yet. Right now we have to focus on saving as many lives as possible."

Authorities had been trying to evacuate those who work in the twin towers, but many were thought to have been trapped. About 50,000 people work at the Trade Center.

"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."

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

Within the hour, an aircraft crashed on a helicopter landing pad near the Pentagon, and the White House, the Pentagon and the Capitol were evacuated.

One of the planes that crashed into the Trade Center was American Airlines Flight 11, hijacked after takeoff from Boston en route to Los Angeles, the airline said. American Airlines issued a statement saying it had "lost" two aircraft Flight 11, with 92 people aboard, and Flight 77 from Washington to Los Angeles, carrying 64 people.

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. The fate of those aboard was not immediately known and it was not clear if the crash was related to the disasters elsewhere. In a statement, the airline also said it was deeply concerned about another plane, Flight 175, a Boeing 767 bound from Boston to Los Angeles.

Authorities went on alert from coast to coast, halting all air traffic, evacuating high-profile buildings and tightening security at strategic installations. The Situation Room at the White House was in full operation.

At the World Trade Center, "everyone was screaming, crying, running, cops, people, firefighters, everyone," said Mike Smith, a fire marshal. "It's like a war zone."

"I just saw the building I work in come down," said businessman Gabriel Ioan, shaking in shock outside City Hall, a cloud of smoke and ash from the World Trade Center behind him. "I just saw the top of Trade Two come down."

Nearby a crowd mobbed a man on 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.

Evacuations were ordered at the United Nations in New York and at the Sears Tower in Chicago. Los Angeles mobilized its anti-terrorism division, and security was intensified around the naval facilities in Hampton Roads, Va.

The planes blasted fiery, gaping holes in the upper floors of the twin towers. A witness said he saw bodies falling and people jumping out. 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.

"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.

The crashes at the World Trade Center happened minutes apart, beginning just before 9 a.m.

Heavy black smoke billowed into the sky above one of New York City's most famous landmarks, and debris rained down on the street, one of the city's busiest work areas. When the second plane hit, a fireball of flame and smoke erupted, leaving a huge hole in the glass and steel tower.

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.

WCBS-TV, citing an FBI agent, said five or six people jumped out of the windows. Witnesses on the street screamed every time another person leaped.

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 was postponed. All bridges and tunnels into Manhattan were closed down.

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.

"A second occurrence is just beyond belief," said Ira Furber, former National Transportation Safety Board spokesman.

"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 New York, "we heard a large boom and then we saw all this debris just falling," said Harriet Grimm, who was inside a bookstore on the World Trade Center's first floor when the first explosion rocked the building.

"The plane was coming in low and ... it looked like it hit at a slight angle," said Sean Murtagh, a CNN vice president, the network reported.

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.