Peter Dicerbo led his 44 colleagues from the First Union National Bank down 47 flights of stairs. He staggered away from the building, his clothes torn; the workers were stunned, dazed and coughing.

Less than 20 minutes later, the United Airlines jet struck the other World Trade Center tower.

"The minute I got out of the building, the second building blew up," said Jennifer Brickhouse, 34, from Union, N.J., who was riding an escalator into the trade center when she "heard this big boom."

"All this stuff started falling," she said, "and all this smoke was coming through. People were screaming, falling and jumping out of the windows."

At least one couple were seen leaping hand-in-hand from the tower's upper stories.

Three miles away, across the East River in Brooklyn, sheets of office paper fluttered out of the sky.

At 9:50 a.m., an hour after the first crash, the first World Trade Center tower collapsed in smoke and rubble.

There were reports of an explosion right before the tower fell, then a strange sucking sound, and finally the sound of floors collapsing. Then came a huge surge of air, followed by a vast cloud of dirt, smoke, dust, paper and debris. Windows shattered. People screamed and dived for cover.

"I heard the largest, loudest collective scream I've ever heard," said Melissa Easton, who was watching from the roof of her Chinatown apartment building about 20 blocks away.

Not long afterward, at 10:30 a.m., the second tower of the World Trade Center collapsed.

The top of the building exploded with smoke and dust. There were no flames, just an explosion of debris, and then more vast clouds swept down to the streets. People were knocked to the ground on their faces as they ran from the building.

Hyman Brown, a University of Colorado civil engineering professor and the construction manager for the World Trade Center, said that flames fueled by thousands of gallons of aviation fuel melted the towers' 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."

In addition to the more than 200 missing firefighters, police officials said nearly 100 of their officers were similarly unaccounted for. Brian Stark, a former Navy paramedic who assisted rescuers, said paramedics had been told that hundreds of police officers and firefighters were missing from the ranks of those sent in to respond to the first crash.

Giuliani said the 2,100 injured included 1,500 "walking wounded" who were taken by boat to New Jersey, and 600 others who were taken to New York hospitals. It could take weeks to dig through the rubble for victims. "I have a sense it's a horrendous number of lives lost," Giuliani said. "Right now we have to focus on saving as many lives as possible."

Hundreds of volunteers and medical workers converged on triage centers, offering help and blood. So many people lined up to donate blood that many were turned away.

The city took on the eerie hush of a metropolis under siege. With public transportation shut down and major bridges and tunnels closed to traffic, walking became the only way to get anywhere. Thousands clogged Manhattan bridges, leaving the city on foot. Throughout the metropolitan area, people stunned by the day's events strolled about as if in a daze.

More than nine hours after the attack, an annex in the complex--7 World Trade Center--continued to burn. At 5:20 p.m., that building collapsed. Blocks away, crowds roared with astonishment.

"People stared open-mouthed and were in shock," said bystander Russ Baker.

Jesus Soriano Jr., 34, of Brooklyn, said he was there when the twin towers fell.