Air temperatures rose to 2,000 degrees in the hottest parts of the fires. Each tower's fire was producing heat equivalent to the power output of a nuclear plant. Jack Daly, the construction manager who heard the helicopter pilot ditch the lightweight floor truss when it was caught in the wind that day in 1970, started to worry that part of the south tower might collapse. Daly says he thought to himself, God, they're going to lose the top. But, he says, ''I never in my world, never, thought the whole thing was going to go down.''

The floor trusses, made of some of the thinnest steel in the World Trade Center, almost certainly began deforming before anything else of consequence. At first, the trusses probably expanded, bowing the exterior columns -- themselves thin and weakening in the heat -- outward in places and causing dangerous stresses. All along the eastern face of the south tower around the 80th floor, tremendous fires raged. Eventually the thin steel of the trusses became so hot in that area that they began to soften and sag, hanging like clotheslines between the exterior and core of the building. The sagging trusses tugged inward on their bolted and welded connections to the exterior columns, and those connections began to snap. Video records of the disaster show a line of dust beginning to blow out of the east face around the 80th floor as floors began to slip away from their moorings and fall one upon the other.

Once Robertson's trusses tore away, the softening exterior columns no longer had anything to keep them from buckling. It was as if two gymnasts standing toe to toe, leaning backward and clasping hands, had suddenly let go. A single column on the east face of the south tower, about 30 feet north of the southeast corner, seems to have been the first to go, according to the videos. As other columns snapped, one by one, the entire top of the building tipped in that direction and, like a tree leaning toward the notch sawed by a lumberjack, began to fall. The force of the upper stories coming down then crushed the entire tower, ripping it apart as it fell. When debris from the top hit the ground, it was moving at an estimated 120 miles an hour. The north tower followed soon after. The death toll would soar to 2,800 people -- many of them,'' a devastated Robertson would write, ''snuffed out by the collapse of structures designed by me.''

Before Sept. 11, Robertson always had an answer for every problem he faced. In conversations after the disaster, he often stammered into silence when trying to explain his feelings about the collapses. Later, he struggled to express himself. ''The responsibility for the design ultimately rested with me,'' Robertson said. ''And I have to ask myself, Should I have made the project more stalwart? And in retrospect, the only answer you can come up with is, Yes, you should have.''

But in other conversations, he became resolute, even defensive. If not for the faraway look in his eyes and the bags underneath them, he could have been the young engineering gunslinger he once was. ''I don't feel blame for not having made it more stalwart than it was,'' he said. ''I don't want to sound egotistical, but maybe it was as good as anyone would have made it, or maybe better than others would have made it. And a lot of that was associated with energy and youth and all that kind of thing. Had it not been me, I think it would have been an older, slower, more accepting kind of person than I was.

''But even so,'' Robertson said, the doubts beginning again, ''had it been more stalwart, surely 1, 2, 50, 100, 1,000 people might have gotten out. It's a big burden. I feel terrible remorse for those who died.''

As the structures weakened and collapsed that morning, David Rockefeller stood at the window of his office in Rockefeller Center, looking south at the smoke billowing over the business district he had done so much to create. He could see the Empire State Building, the old rival to the towers, in the foreground, a little to the east. When Rockefeller was a child, his family lived just a few streets north of where his office is now, and his mother commissioned a painting of the view from his bedroom window. The painting, which still hangs a few feet from where Rockefeller watched as the weakened steel lost its grip, depicts the 1930's New York skyline to the south, a jumble of lower buildings dominated by the Empire State Building to the east. On the West Side of the city, there is only a great swath of sky running the length of the island.

''There was so much smoke that we didn't really, fully understand -- the buildings literally collapsed,'' Rockefeller says. ''You could see something drastic was happening, but it was so horrible in a way that it was almost like a dream, a bad dream.'' When the smoke thinned, he saw again the swath of empty sky in the west, much as it once appeared from his bedroom window. But now even the sky had been invested with a horrible meaning. History had been undone.