In an unprecedented barrage of terrorist attacks, both towers of the World Trade Center collapsed after being struck by two different hijacked planes Tuesday morning, and authorities have evacuated Lower Manhattan amid a cloud of choking debris to search for survivors.
A commercial airliner crashed into the first tower shortly before 9 a.m., and a second plane struck the other tower about 15 minutes later.
A third plane struck the Pentagon 40 minutes after the attack in New York, destroying one of the fives sides of the nation's military headquarters and causing extensive casualties. A fourth plane crashed near Pittsburgh, Pennsylvania.
Two World Trade Center, which was hit second, completely collapsed nearly an hour after the initial impact, sending thousands of panicked people on the street running for cover as a cloud of debris engulfed Lower Manhattan. One World Trade Center crumbled to the ground about 30 minutes later, completely erasing a landmark from the New York City skyline.
The full extent of the casualties is not yet known.
Rescue crews were on the scene soon after the plane crashes and may not have been aware that the structures were unstable, since the damage and fire seemed to be isolated at the top of the towers following the crashes.
Around eight hours later, at 5:30 p.m., the 47-story Seven World Trade Center collapsed as well, and other surrounding buildings are said to be structurally unsound and in danger of falling. Seven World Trade had housed the Office of Emergency Management's multi-million dollar bunker, which would have been the coordination center for the massive rescue operation in such a crisis.
Eighty miles southeast of Pittsburgh, meanwhile, United Airlines Flight 93 crashed en route to San Francisco from Newark Monday morning. A passenger had called authorities on his cellphone to say he was locked in the bathroom and that the plane had been hijacked.
American Airlines Flight 11 from Boston to Los Angeles was hijacked Monday morning and is thought to have been the first to crash into the World Trade Center. The second plane is thought to be United Airlines Flight 175, also from Boston to Los Angeles. The plane that hit the Pentagon, American Flight 77, had taken off from Washington's Dulles airport on its way to San Francisco.
A total of 266 people were on board the four crashed flights.
The White House was evacuated after terrorist threats were received and all other federal offices were cleared as a precaution as well.
President Bush was in Florida at the time of the crashes and then flew to an Air Force base in Louisiana temporarily for safety reasons. "We will do what it takes, whatever is necessary to secure America and Americans," said Bush, who said the country would find and punish those responsible.
So far no one has claimed responsibility for the attacks.
In an unprecedented move, the FAA has grounded all flights nationwide until at least noon on Wednesday in response to the attacks. Most international flights on their way to the U.S. were directed to Canada to land.
The U.S. Navy is also sending two aircraft carriers to New York City to provide air cover in precaution against any further attack.
The city is under a state of terrorist alert. Bridges and tunnels are only open to outbound traffic from Manhattan, with no vehicles allowed back in. Most subway service in Manhattan has been restored after an earlier shutdown, but several lines are still closed.
Authorities urged the evacuation of all buildings in Lower Manhattan below Canal Street after the collapse in order to clear the area for 10,000 rescue personnel on the scene. Tens of thousands of people, many of them wearing masks, headed north to get away from the thick soot in the air.
"I have a sense it's a horrendous number of lives lost," said Mayor Rudolph Giuliani when asked about the number of victims. "Right now we have to focus on saving as many lives as possible."
The National Guard has been called in to assist city rescue personnel.
The New York City primary election scheduled for Tuesday has been postponed and polls are closed.
It appeared that the situation was contained after the first explosion at World Trade Center Tower 1, when the crash of the second plane was broadcast over live television around 15 minutes later. The impact seemed contained again, and people stood on the street watching the smoke for almost an hour before the sudden and unexpected collapses.
Immediately after the plane crashes, witnesses reported seeing a plane's wheel as well as shoes from some victims fall to the ground several blocks from the towers. Fire burned in both towers until the collapse, black smoke billowing out of the gaping holes left in each building by the planes' impact.
The towers were evacuated after the crashes but an unknown number of people were still trapped inside when the buildings crumbled. People could be seen jumping from windows in the skyscrapers to certain death before the collapse.
Approximately 54,000 people worked in the 110-story World Trade Center and an average of 150,000 people in all pass through the office complex and its underground concourse daily. It is still unclear how many people were present at the time of the initial attack or collapse, but an unnamed police official told the Associated Press that the the number of casualties will likely be in the thousands.
The vice president of the city firefighters' union, Mike Carter, estimated that 200 of the first 400 firefighters on the scene died. "We have entire companies that are just missing," he said. "We're going to have to bury a lot of people."
Hundreds of people hit by flying debris and other injured flooded nearby hospitals for treatment. Nearly 1,500 more were treated for less serious injuries.
Bodies were scattered over the scene of the collapse and buried in two feet of rubble that covers surrounding streets.
The attacks come eight years after the World Trade Center was attacked by terrorists in a bombing that killed six people and injured more than 1,000 others in 1993. One of the bombers convicted in that attack was set to be sentenced Wednesday.
The attacks also took place near the anniversary of the 1978 Camp David peace accords, fueling speculation that the attacks could be related to the violence in the Mideast.
Word of the attacks sparked cheering and celebration among small groups of Palestinians in East Jerusalem. Palestinian leader Yasir Arafat, however, publicly condemned the attacks.
Afghanistan's Taliban rulers condemned the attacks in New York and Washington, saying accused terrorist mastermind Osama bin Laden - a fugitive from U.S. justice hiding within their borders - was not behind them.
A London-based journalist says bin Laden's followers warned three weeks ago that they would carry out a huge and unprecedented attack on U.S. interests. He says he didn't take the threat seriously because such warnings are often given but not carried out.
The crashes are being called by many the worst attack on U.S. soil since Japan's bombing of Pearl Harbor.