Q: What caused the "squibs"? Could they have been just puffs of dust being pushed out of the Towers by falling floors? Are they visual evidence of explosive charges?

A: The isolated ejections 20-60 stories below the demolition front appear to be composed of pulverized building materials, including concrete. There was no known mechanism by which pulverized building materials being created up at the zone of destruction could have been transported so far down through the building and to the exterior. Air conditioning vents would not have tolerated such pressures, and there was no other "channel" in the building to deliver “compressed air”.

There is no reason, on the “dust puff” theory, for such blasts to be as isolated as they were. Massive air pressure which would delivered by the (missing) “pile driver” down through the elevator hoist ways and out through a given floor would have broken most or all windows on that floor – not created the highly focalized pin-point ejections that are seen on the videos. The breakage of one or two windows on a given floor would not have relieved enough pressure across an entire floor area to prevent the breakage of many other windows nearby.

Images like this one reveal that the squibs were not
merely puffs of air, as they have the same hue
and consistency as pulverized solid building materials.

Another problem with the “dust puff” theory is that the pulverized building materials would not have been transported so quickly. Air would have been pushed ahead of such materials, resulting in transparent puffs of air flowing through the freshly broken windows.

Physicist David Chandler has also shown that some of these ejections came from the corners of the buildings. Since there are no windows on the corners, these ejections could not have been the result of air pressure.

Furthermore, calculations performed by Dr. Crockett Grabbe show that the horizontal ejection rate of the squibs is disproportional to the floor and debris descent speeds that are allegedly responsible for them. As Kevin Ryan has shown, the ejection speed of the squibs from the Towers also matches the speeds recorded for ejections caused by explosives.

As to these ejections appearing only after the collapse initiation, it should be noted that the North Tower's antenna dropped before any other building movement is seen, which is evidence that demolition devices were working on the core before any squibs were seen emerging out of the perimeter walls. There is also video showing that some of these ejections occurred even before the collapses began. See "Visible Explosion at World Trade Center" and "WTC1 collapse initiation - visible signs.