Squibs. Can you prove without a doubt that those were caused by explosives and not air pressure?
It is NIST that has to prove their theory since they were charged with that task, and there is sufficient counter theory to doubt their explanation.
Defenders of the gravity collapse theory consistently invoke the explanation that the ejections of dust are caused by pancaking floors squeezing out air and dust. The Popular Mechanics article attacking 9/11 Truth contains the following passage.
Like all office buildings, the WTC towers contained a huge volume of air. As they pancaked, all that air--along with the concrete and other debris pulverized by the force of the collapse--was ejected with enormous energy. "When you have a significant portion of a floor collapsing, it's going to shoot air and concrete dust out the window," NIST lead investigator Shyam Sunder tells PM. Those clouds of dust may create the impression of a controlled demolition, Sunder adds, "but it is the floor pancaking that leads to that perception."
NIST's final Report on the Twin Towers mentions the piston theory to attempt to explain away the ejections:
The falling mass of the building compressed the air ahead of it, much like the action of a piston, forcing material, such as smoke and debris, out the windows as seen in several videos.
There are several problems with this explanation, which we designate the piston theory.
* The squibs contain thick dust of a light color, apparently from crushed concrete and gypsum.
But these materials would not have been crushed until the pancaking floors above impacted the floor emitting the squib. Thus the dust would not be produced until the air was already squeezed out, so there was no source of the dust for the squib.
* The squibs emerge from the facade
10 to 20 floors below the exploding rubble cloud inside of which the tower is disintegrating. The thick clouds appear to contain the pulverized concrete of the floor slabs, which was the only concrete component of the tower. But the piston theory requires that the floors have already pancaked down to the level of the squib, making them unavailable for the production of the concrete dust more than 10 floors above.
* The piston theory requires a rather orderly pancaking of the floor diaphragms within the intact sleeve of the perimeter wall. Such a process should have left a stack of floor diaphragms at the tower's base at the end of the collapse.
But there was no such stack. In fact, it is difficult to find recognizable pieces of floor slabs of any size in Ground Zero photographs.
* The North Tower exhibits three distinct sets of squibs at different elevations of the building. Each set is visible as two distinct squibs on the same floor, one emerging from about the horizontal center of each of the tower's two visible faces.
This pattern is is far too focused and symmetric to be explained by the piston theory, which would produce similar pressures across each floor and over successive floors.
* The pancaking of floors within the perimeter wall would have created under pressures in the region above the top pancaking floor. But we seen no evidence of dust being sucked back into the tower.
9-11 Research: Squibs