Professor Emeritus of Physics: The Thermal and Gravitational Energy Available Were Insufficient to Melt Steel in the Twin Towers and 7 World Trade Center on 9/11/01 [pdf]

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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.
One tank holding 6,000 gallons of fuel was in the building to provide power to the command bunker on the 23rd floor. Another set of four tanks holding as much as 36,000 gallons were just below ground on the building's southwest side for generators that served some of the other tenants.
Engineers and other experts have already uncovered evidence at the collapse site suggesting that some type of fuel played a significant role in the building's demise, but they expect to spend months piecing together the picture of what remains a disturbing puzzle.

http://query.nytimes.com/gst/fullpage.html?res=9E02E3DE143DF93AA15752C1A9679C8B63