NEW YORK -- In a horrific sequence of destruction, terrorists crashed two planes into the World Trade Center and the twin 110-story towers collapsed Tuesday morning. Explosions also rocked the Pentagon and the State Department and spread fear across the nation.
The fate of those in the skyscrapers was not immediately known. Authorities had been trying to evacuate the 50,000 people who work in the twin towers, but many were thought to be trapped.
"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, a car bomb exploded outside the State Department, and the West Wing of the White House was evacuated amid threats of terrorism. And another explosion rocked New York about an hour after the crash.
Authorities went on alert from coast to coast, halting all air traffic, evacuating high-profile buildings and tightening security at strategic installations.
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.
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.
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.
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," said Keith Meyers, 39, of 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.