Establishing the U.S. death toll could take weeks. The four airliners alone had 266 people aboard and there were no known survivors. At the Pentagon, about 100 people were believed dead.
In addition, a union official said he feared half of the 400 firefighters who first reached the scene had died in rescue efforts at the trade center - where 50,000 people worked - and dozens of police officers were believed missing.
``The number of casualties will be more than most of us can bear,'' a visibly distraught Mayor Rudolph Giuliani said.
Police sources said some people trapped in the twin towers managed to call authorities or family members and that some trapped police officers made radio contact. In one of the calls, which took place in the afternoon, a businessman phoned his family to say he was trapped with policemen, whom he named, the source said.
Aided by an intercept of communications between his supporters and harrowing cell phone calls from at least one flight attendant and two passengers aboard the jetliners before they crashed, U.S. officials began assembling a case linking bin Laden to the devastation.
At the World Trade Center, the dead and the doomed plummeted from the skyscrapers, among them a man and woman holding hands.
Shortly after 7 p.m., crews began heading into ground zero of the attack to search for survivors and recover bodies. All that remained of the twin towers by then was a pile of rubble and twisted steel that stood barely two stories high, leaving a huge gap in the New York City skyline.
The first airstrike - on the trade center - occurred shortly before 8:45 a.m. EDT. A burning, 47-story part of the trade center complex, long since evacuated, collapsed in flames just before nightfall.
Emergency Medical Service worker Louis Garcia said initial reports indicated that bodies were buried beneath the two feet of soot on streets around the trade center.
``A lot of the vehicles are running over bodies because they are all over the place,'' he said.
Said National Guard member Angelo Otchy of Maplewood, N.J., ``I must have come across body parts by the thousands. I came across a lady, she didn't remember her name. Her face was covered in blood.''
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.
Bridges and tunnels were closed to all but pedestrians. Subways were shut down for much of the day; many commuter trains were not running.
Giuliani said it was believed the aftereffects 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.
Well into the night, a steady stream of boats continued to arrive in the park. ``Every 10 minutes another boat with 100 to 150 people on it pulls up,'' said Mayor Glenn Cunningham. ``I have a feeling this is going to go on for several days.''
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