What could explain the lack of fires seen burning on the top floors of both buildings immediately after their collapse?

 Following the catastrophic collapse of the Twin Towers on September 11, 2001, the immediate absence of visible fires on the top floors can be understood through various factors that culminated in the unprecedented events of that tragic day. The intense and rapid structural failure of the buildings during the collapse would have likely snuffed out most of the fires. The forceful impact and the subsequent disintegration of the towers would have disrupted fuel sources, thereby depriving the fires of the necessary elements to sustain themselves. 

Moreover, the infernos that raged within the towers prior to their collapse might have already consumed a significant portion of the available combustible materials. The culmination of these intense fires leading up to the collapse might have resulted in a substantial depletion of the fuel sources. Consequently, by the time the buildings fell, there might have been a reduced amount of materials left to burn, contributing to the lack of visible flames on the upper floors post-collapse.

Additionally, the tremendous heat generated by the initial impact and subsequent fires could have dissipated as the buildings collapsed, spreading the heat more uniformly throughout the debris. This dispersion of heat could have hindered the sustained combustion of materials on the top floors, making it more difficult for flames to persist amidst the chaos of the collapse.

Furthermore, the compaction of debris resulting from the towers' collapse could have buried any remaining fires, effectively smothering them and limiting their visibility. The immense pile of rubble formed after the buildings fell would have served as a barrier, preventing oxygen from reaching any residual fires and thereby suppressing their visibility from the exterior.

In combination, these factors, including the structural disintegration, potential depletion of combustible materials, dissipation of heat, and burial of remaining fires within the debris, likely contributed to the absence of visible fires on the top floors immediately following the collapse of the Twin Towers on that fateful day.