Molten Steel Poured From Upper Floors Before Collapse
The official NIST report noted the presence before the building collapse of an "unusual flame" burning white hot and producing white smoke in the photo below. A white flame is indicative of extreme high temperature which would be achievable by incendiaries like thermite but certainly not by jet fuel or office furnishings burning in air:
Some have suggested that the flowing molten metal could have been aluminum from office furnishings and plane wreckage (that must have somehow been heated in some kind of container that spontaneously formed and then dumped its contents out the windows) but the problem with that idea is that molten aluminum does not glow reddish when molten, for two reasons: 1) it melts at lower temperatures (which could almost conceivably be achieved by jet fuel and office furnishings) but does not glow red at such low temperatures, and 2) aluminum's properties being different from iron and steel (see below) causes it to not glow red even at temperatures where iron or steel might. Below is a photograph of molten aluminum being poured so that you can see it shows no trace of a red glow.
Metal Temperature by Color
The text and temperature graphic below were taken from Dr. Steven Jone's paper Why Indeed Did the WTC Buildings Completely Collapse?.
The following table (see http://www.processassociates.com/process/heat/metcolor.htm ) provides data regarding the melting temperatures of lead, aluminum, structural steel and iron, along with approximate metal temperatures by color. Note that the approximate temperature of a hot metal is given by its color, quite independent of the composition of the metal. (A notable exception is falling liquid aluminum, which due to low emissivity and high reflectivity appears silvery-gray in daylight conditions, after falling through air 1-2 meters, regardless of the temperature at which the poured-out aluminum left the vessel. Aluminum does incandesce (glow) like other metals, but faintly, so that with the conditions described in the previous sentence (which prevailed at the WTC on 9/11), falling liquid aluminum will appear silvery-gray. Rapid oxidation of the hot flowing aluminum will contribute to the observed appearance. [Experiments: Jones, 2006])
For comparison, the official NIST study which examined WTC steel that had actually been exposed to fire at higher floors near the points of impact found that the maximum temperaturess achieved were about 600 degrees C., far lower than necessary to melt steel or even soften it to the point of structural failure. No explanation for this apparent discrepancy with the official conspiracy theory was offered.