Almost lost in the chaos of the collapse of the World Trade Center is a mystery that under normal circumstances would probably have captured the attention of the city and the world. That mystery is the collapse of a nearby 47-story, two-million-square-foot building seven hours after flaming debris from the towers rained down on it, igniting what became an out-of-control fire.

Engineers and other experts, who quickly came to understand how hurtling airplanes and burning jet fuel had helped bring down the main towers, were for weeks still stunned by what had happened to 7 World Trade Center. That building had housed, among other things, the mayor's emergency command bunker. It tumbled to its knees shortly after 5:20 on the ugly evening of Sept. 11.

The building had suffered mightily from the fire that raged in it, and it had been wounded by the flying beams falling off the towers. But experts said no building like it, a modern, steel-reinforced high-rise, had ever collapsed because of an uncontrolled fire, and engineers have been trying to figure out exactly what happened and whether they should be worried about other buildings like it around the country.

As engineers and scientists struggle to explain the collapse of 7 World Trade Center, they have begun considering whether a type of fuel that was inside the building all along created intensely hot fires like those in the towers: diesel fuel, thousands of gallons of it, intended to run electricity generators in a power failure.