For those in the UK, watch the video on channel 5's My5 app -- "Conspiracy: twin towers: the missing evidence". It is conjectured that the true cause was aluminium from the crashed aircraft, which, contrary to NIST's findings, didn't disintegrate. Instead, it heated up to around 1000 deg. C because it was encased in insulating material from the floors (mimicking an aluminium smelter), and there were plenty of accelerants like Iron oxide lying around.
When water is added to molten aluminium, it can explode, which would account for explosions heard by firefighters. Once you have an explosion, pancaking of floors beneath the explosion could occur. There were 30,000 gallons of water in the buildings in sprinkler systems alone, which latter the crash could have damaged to release many gallons rather than mere sprinkles.
Any way, watch the video and make up your own mind. Interestingly, the NY port authority, where building fragments are stored, have refused access to them to check the hypothesis, and although NIST noted the cascades of molten material were probably aluminium, they haven't budged from their past assessment. Which was, that the cause was insulating material being dislodged from steel supporting girders, causing the steel to have weakened by the heat much sooner than otherwise.