Here is a sequence of photos of WTC1 collapse, which was the second WTC building to collapse but the first building to be hit by an airplane. The fires were in floors 94 through 98. From the 2005 NIST report and photos, the first floor to collapse was 97. Prior to the collapse, there were no fires or damage to more than 90 floors below. The smoke near the ground in the far left photo was from the collapse of WTC 2 which had already collapsed.
Shown in these photos, WTC1 collapses very rapidly and the timing of the collapse is only 0.8 seconds more than an object falling in free fall in a vacuum (i.e. no resistance to the fall) from height equal to the top of this building, 110 stories. For this building, or any object, to fall at near free fall rate of acceleration, it cannot meet any resistance to its fall. Any resistance would slow the fall.
In free fall, the floors cannot have impacted the floors below, which is in fact what happened. The official revised government report from NIST acknowledges the building was in near free fall, as do many engineers and architects. The floors fall together and accelerate downward together until they finally meet resistance from the structure and ground below.
If the floors were pancaking on top of each other, collapsing down vertically on top of each other, that impact could have produced massive dust and smoke clouds since the concrete, wall board, desks, etc would be crushed by the massive weight and momentum of the falling mass from above. But, the photos, video and timing definitively show that pancaking did not happen. NIST and other official reports have backed away from the pancaking theory. Based on timing confirmed by multiple videos from different angles, the falling building met no resistance to its fall, even though the lower 90 floors were undamaged by the airplane or weakened by fires.
So, beginning in the second photos from the left, followed by the 3rd and 4th photos, what caused the smoke and dust?