NEW YORK (AP) - In the most devastating terrorist onslaught ever waged against the United States, hijackers crashed two airliners into the World Trade Center on Tuesday, toppling its twin 110-story towers. The deadly calamity was witnessed on televisions across the world as another plane slammed into the Pentagon, and a fourth crashed outside Pittsburgh.
Said Adm. Robert J. Natter, commander of the U.S. Atlantic Fleet: "We have been attacked like we haven't since Pearl Harbor."
Establishing the U.S. death toll could take weeks, but it was expected casualties would be in the thousands. The four airliners alone had 266 people aboard and there were no known survivors.
In addition, a firefighters union official said he feared an estimated 200 firefighters alone had died in rescue efforts. A police source, speaking on condition of anonymity, said 78 officers were missing.
The dead and the doomed plummeted from the skyscrapers, among them a man and woman holding hands.
"Freedom itself was attacked this morning and I assure you freedom will be defended," said President Bush, who was in Florida at the time of the catastrophe. As a security measure, he was shuttled to a Strategic Air Command bunker in Nebraska before leaving for Washington.
"Make no mistake," he said. "The United States will hunt down and pursue those responsible for these cowardly actions."
No one took responsibility for the audacious events that rocked the seats of finance and government, but federal authorities identified Osama bin Laden _ who has been given asylum by Afghanistan's Taliban rulers _ as the prime suspect.
More than nine hours after the U.S. attacks began, explosions could be heard north of the Afghan capital of Kabul, but American officials said the United States was not responsible. "It isn't us. I don't know who's doing it," Pentagon spokesman Craig Quigley said.
The nation's aviation system was shut down as officials considered the frightening flaws that had been exposed in security procedures.
Barbara Olson, the wife of U.S. Solicitor General Theodore Olson, used a cell phone to tell her husband that her plane had been hijacked by attackers using knives and sharp instruments. The plane carrying Mrs. Olson, a frequent CNN commentator, crashed into the Pentagon.
Officials across the world condemned the attacks but in the West Bank city of Nablus, thousands of Palestinians celebrated, chanting "God is Great" and handing out candy.
At the Pentagon, the symbol and command center for the nation's military force, one side of the building collapsed as smoke billowed over the Potomac River. Rep. Ike Skelton, briefed by Pentagon officials, said, "There appear to be about 100 casualties" in the building.
The television images were extraordinary: a plane slamming into the second tower as smoke poured from the first; the buildings tumbling down and vanishing in a gray cloud; bloodied survivors stumbling through the streets of Manhattan, covered with dust and ashes.
The attacks altered the very skyline of Manhattan, destroying two buildings where 50,000 people worked. The first airstrike occurred shortly before 8:45 a.m. EDT. By evening, huge clouds of smoke still billowed from the ruins. A burning, 47-story part of the World Trade Center complex collapsed in flames just before nightfall. The building had already been evacuated.
Mike Carter, vice president of the city's firefighters union, estimated that half of the 400 firefighters who first reached the scene may be dead. "We have entire companies that are just missing," he said. "We lost chiefs ... We're going to have to bury a lot of people."
The violence sent waves of fear across the continent and beyond. The aviation shutdown was the first in history. Financial markets were closed, too.
Top leaders of Congress were led to an undisclosed location as were key officials of the Bush administration. Guards armed with automatic weapons patrolled the White House grounds and military aircraft secured the skies above the capital city.
Evacuations were ordered at the tallest skyscrapers in several cities, and high-profile tourist attractions closed _ Walt Disney World, Mount Rushmore, Seattle's Space Needle, the Gateway Arch in St. Louis.
"No one has been ruled out, but our initial feeling is that this is the work of bin Laden," said a high-ranking federal law enforcement official who spoke on condition of anonymity. "He is top of our list at this point."
In Afghanistan, where bin Laden has been given asylum, the nation's hardline Taliban rulers rejected suggestions he was responsible.
Bin Laden came to prominence fighting alongside the U.S.-backed Afghan mujahedeen _ holy warriors _ in their war against Soviet troops in the 1980s. But former followers say he turned against the United States during the 1991 Gulf War, seething at the deployment of U.S. troops in Saudi Arabia during the Gulf War campaign to oust Iraq from Kuwait. He has repeatedly called on Muslims worldwide to join in a jihad, or holy war, against the United States.
Abdel-Bari Atwan, editor of the Al-Quds al-Arabi newspaper, said he received a warning from Islamic fundamentalists close to bin Laden, but had not taken 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.
Eight years ago, the World Trade Center was a terrorist target when a truck bomb killed six people and wounded about 1,000 others. Tuesday's attack was far more deadly, and sure to be well above the 168 people killed in the 1995 bombing of the federal building in Oklahoma City.
This is how Tuesday's mayhem unfolded:
At about 8:45 a.m., a hijacked airliner crashed into the north tower of the trade center, the 25-year-old, glass-and-steel complex that was once the world's tallest.
Clyde Ebanks, an insurance company vice president, was at a meeting on the 103rd floor of the south tower when his boss said, "Look at that!" He turned to see a plane slam into the other tower.
"I just heard the building rock," said Peter Dicerbo, a bank employee on the 47th floor. "It knocked me on the floor. It sounded like a big roar, then the building started swaying. That's what really scared me."
The enormity of the disaster was just sinking in when 18 minutes later, the south tower also was hit by a plane.
"All this stuff started falling and all this smoke was coming through. People were screaming, falling, and jumping out of the windows," said Jennifer Brickhouse, 34, from Union, N.J.
The chaos was just beginning. Workers stumbled down scores of flights, their clothing torn and their lungs filled with smoke and dust.
John Axisa said he ran outside and watched people jump out of the first building; then there was a second explosion, and he felt the heat on the back of his neck.
Donald Burns, 34, was being evacuated from the 82nd floor when he saw four people in the stairwell. "I tried to help them but they didn't want anyone to touch them. The fire had melted their skin. Their clothes were tattered," he said.
Worse was to come. At 9:50, one tower collapsed, sending debris and dust cascading to the ground. At 10:30, the other tower crumbled.
Glass doors shattered, police and firefighters ushered people into subway stations and buildings. The air was black, from the pavement to the sky. The dust and ash were inches deep along the streets.
Mayor Rudolph Giuliani said it was believed the after effects of the plane crashes eventually brought the buildings down, not planted explosive devices.
Hyman Brown, a University of Colorado civil engineering professor and the construction manager for the World Trade Center, speculated that flames fueled by thousands of gallons of aviation fuel melted steel supports.
"This building would have stood had a plane or a force caused by a plane smashed into it," he said. "But steel melts, and 24,000 gallons of aviation fluid melted the steel. Nothing is designed or will be designed to withstand that fire."
At mid-afternoon, Giuliani said 1,500 "walking wounded" had been shipped to Liberty State Park in New Jersey by ferry and tugboat, and 750 others were taken to New York City hospitals, among them 150 in critical condition.
Bystanders helped the injured, along with multitudes of emergency workers who poured into the area. It was feared that many police and firefighters were in the two buildings when they went down.
Bridges and tunnels were closed to all but pedestrians. Subways were shut down; commuter trains were not running.
Meanwhile, at about 9:30 a.m., an airliner hit the Pentagon _ the five-sided headquarters of the American military. "There was screaming and pandemonium," said Terry Yonkers, an Air Force civilian employee at work inside the building.
The military boosted security across the country to the highest levels, sending Navy ships to New York and Washington to assist with air defense and medical needs.
A half-hour after the Pentagon attack, a United Airlines Flight 93, a Boeing 757 jetliner en route from Newark, N.J., to San Francisco, crashed about 80 miles southeast of Pittsburgh. A congressman said the hijackers intended to send the plane to crash into Camp David, the presidential retreat in Maryland.
Minutes before the crash, a passenger told an emergency dispatcher in a cell phone call: "We are being hijacked, we are being hijacked!"
Airline officials said the other three planes that crashed were American Airlines Flight 11, a Boeing 767 from Boston to Los Angeles, apparently the first to hit the trade center; United Airlines Flight 175, also a Boeing 767 from Boston to Los Angeles, which an eyewitness said was the second to hit the skyscrapers; and American Airlines Flight 77, a Boeing 757 en route from Washington-Dulles to Los Angeles that a source said hit the Pentagon.
The nightmare sent shockwaves across the country. The Federal Reserve, seeking to provide assurances that the nation's banking system would be protected, said it would provide additional money to banks if needed.
The Department of Health and Human Services said 7,000 doctors and other health professionals were ready to help if needed.
"We're at war," said Gaillard Pinckney, an employee at the Housing and Urban Development office in Columbia, S.C. "We just don't know with who."
Felix Novelli, who lives in Southampton, N.Y., was in Nashville with his wife for a World War II reunion. He was trying to fly home to New York when the attacks occurred
"I feel like going to war again. No mercy," he said. "This is Dec. 7th happening all over again. We have to come together like '41, go after them."
The attack on Pearl Harbor claimed the lives of 2,390 Americans, most of them servicemen.