http://www.pbs.org/wgbh/nova/transcripts/2907_wtc.html
NARRATOR: Leslie Robertson's radical design seemed to have worked, but there was more devastating damage hidden inside. Although the aluminum aircraft shattered on contact with the exterior wall, the speed and force of the fragments and the intact steel engines severely damaged the columns and stairwells in the core, and jet fuel began saturating the building.
PAUL NEAL: Almost immediately after the impact, somewhat bizarrely, I smelled an overwhelming stench of aviation fuel, Jet A1 gas, which I recognized because I'm a private pilot and I'm used to airfield environments. I recall smelling it and almost instantly dismissed it as being illogical and didn't have any place in the World Trade Center.
NARRATOR: In an instant, the fuel ignited a massive fire that quickly engulfed the damaged area, and this was something even Robertson had not considered.
LESLIE ROBERTSON: With the 707, to the best of my knowledge, the fuel load was not considered in the design. Indeed, I don't know how it could have been considered.
CHARLES THORNTON: They didn't have the mathematical models in the computers to model a fire as a result of the fuel in a 707. I was asked in 1986 what would happen if a plane flew into the Trade Center. And I said it would not knock the building down from the pure physics of the mass hitting the building. But we...none of us really focused on that kind of a fuel fire.