9/11 Explosion according to NIST: WTC South Tower 9:37 am

“The video shot from the WTC plaza captured an intriguing event at 9:37:04 a.m. A jet of air, dust, and a large piece of debris was ejected from a window, 77-355, on the 77th floor at an extremely high velocity. Longer distance videos show that puffs of smoke and or dust appeared simultaneously on the east face from several open windows near the center of the 78th floor and from open windows on the north side of the 79th floor.” – Page 50/268 in NIST NCSTAR1-5A Ch 9 Appx C. Their theory of why it happened is on page 55/268, third paragraph:

“It is conceivable that the pressure pulse generated within WTC 2 at 9:37:04 a.m. could have resulted from the sudden collapse of the hanging object, presumed to be a portion of the 83rd floor slab, observed through open 82nd floor windows. While the rapid collapse of a large secion of a floor slab would likely generate a substantial pressure pulse, evidence supporting such a conclusion is circumstantial, at best. The actual collapse of the object was not observed, and there is no evidence that it occurred rapidly or even at this time. The appearance of large fires on the 82nd floor shortly after the pressure pulse is an indication that something happened on this floor around this time. On the other hand, most of the direct effects of the pressure pulse were observed on lower floors, particularly floors 77 through 79. The available pathways for transporting the pressure pulse are unknown, but it might have been expected that its largest effects would have been apparent near the floor where it was generated. It must be concluded that, while it is possible that the pressure pulse resulted from a partial collapse of the hanging object seen on the east side of the tower, the evidence is far from conclusive.” – NIST NCSTAR 1-5A Ch. 9 Appx. C, p.55/268.

In other words, NIST does not know what caused the explosion.