The evidence strongly suggests that 1,100 or more people in or above the impact zones survived the initial crashes, roughly 300 in the south tower and 800 in the north. Many of those lived until their building collapsed.

Even after the second airplane struck, an open staircase connected the upper reaches of the south tower to the street. The Times has identified 18 men and women who used it to escape from the impact zone or above. At the same time they were evacuating, at least 200 other people were climbing toward the roof in that tower, unaware that a passable stairway down was available, and assuming -- incorrectly -- that they could open the roof door. ''The belief that they had a rooftop option cost them their lives,'' said Beverly Eckert, whose husband, Sean Rooney, called after his futile trek up.

Hundreds were trapped on floors untouched by the airplanes. Even though the buildings survived the initial impacts, the twisting and bending of the towers caused fatal havoc. Stairwells were plugged by broken wallboard. Doors were jammed in twisted frames. With more time and simple tools like crowbars, rescue workers might have freed people who simply could not get to stairways. In the north tower, at least 28 people were freed on the 86th and 89th floors by a small group of Port Authority office workers who pried open jammed doors. Those self-assigned rescuers died.

In both towers, scores of people lost chances to escape. Some paused to make one more phone call; others, to pick up a forgotten purse; still others, to perform tasks like freeing people from elevators, tending the injured or comforting the distraught.

The crises had identical beginnings and endings in each tower, but ran different courses. At least 37 people, and probably well over 50, can be seen jumping or falling from the north tower, while no one is visible falling from the south tower, in a collection of 20 videotapes shot by amateurs and professionals from nearby streets and buildings. Both towers had similar volumes of smoke and heat, but in the north tower, about three times as many people were trapped in roughly half the space. Scores were driven to the windows of the north tower in search of relief. In the south tower, people had more opportunities to move between floors.

The impact zones formed pitiless boundaries between those who were spared and those who were doomed. Even at the margins, the collisions were devastating: the wingtip of the second plane grazed the 78th floor sky lobby in the south tower, instantly killing dozens of people waiting for elevators. In all, about 600 civilians died in the south tower at or above the plane's impact. In the north tower, every person believed to be above the 91st floor died: 1,344.

The farther from the impact, the more calls people made. In the north tower, pockets of near-silence extended four floors above and one floor below the impact zone. Yet remarkably, in both towers, even on floors squarely hit by the jets, a few people lived long enough to make calls.