Knowledge that the burning temperatures of jet fuel would not melt the steel support structure of the WTC contributed to the belief among skeptics that the towers would not have collapsed without external interference (something other than the planes). NIST does not claim that the steel was melted, but rather that the weakened steel, together with the damage caused by the planes' impacts, caused the collapses. NIST reported that a simulation model based on the assumption that combustible vapors burned immediately upon mixing with the incoming oxygen showed that "at any given location, the duration of temperatures near 1,000 °C was about 15 to 20 [minutes. The rest of the time, the calculated temperatures were 500 °C or below."