Way back in 2007, a physics professor at Brigham Young University, Dr Steven E. Jones, turned up critical evidence while investigating samples of World Trade Center (WTC) dust. The samples had been collected immediately after September 11, 2001 from the thick deposit of dust that blanketed the WTC site and much of lower Manhattan. Jones found tiny bits of an exotic incendiary known as thermate that can cut through steel like a hot knife through butter. Thermate burns at ~5,000°F. The main product of the reaction is molten iron. Thermate differs from its better known cousin thermite in that it contains sulfur which lowers the melting point of iron, speeding up the reaction. The presence of both sulfur and aluminum was diagnostic for thermate. Jones called this "the last nail in the coffin." (Dr. Steven E. Jones, Revisiting 9/11/2001. Applying the Scientific Method, 2007, posted here)