The above broadcast video of the WTC-2 collapse has high enough resolution that, when viewed frame by frame, a series of small bright flashes can be seen during the collapse.  Most of them last from for only one frame (1/30 sec.) and appear as bright white point sources superimposed on the outlines of the building.   At first these might be taken for video noise, but the fact that they only occur in areas occupied by the South Tower and are bigger than a single pixel argues against this.  The flashes are very similar in brightness and high color temperature, exhibiting a blue-white spectrum consistent with the light emitted by high explosives.  The flashes also show a dark halo around the central bright spot.  This is an artifact created in the detector element of the video camera around small bright objects, and further indicates that these flashes are real events.

If you want to look at this phenomenon you will need to download the above video (right-click and select "Save Target As...").  It can then be viewed frame-by-frame with a video player program or frames can be extracted and viewed singly.  These flashes are plainly seen, especially if viewed in full-screen mode.

The first of these flashes is much longer than the rest, and can be seen in the above picture near the far end of the darker north wall, at the same height as the first row of explosions.  It lasts for a full second (30 frames) and is finally obscured by the growing dust cloud.  This looks more like a small intense jet or flare and is qualitatively different from the briefer flashes seen later.  It could possibly be thermite or another relatively slow-burning explosive.

Soon after the beginning of this longer first flash the first of the brief flashes is seen, about one third of the way way up along the tower's northeast corner (the one facing us), and slightly to the right of the actual corner.  Notice that this first brief flash takes place just before the upper section begins to telescope on itself, and the multiple flashes near the end of the clip take place just ahead of the collapse wave, exactly as would be needed to allow the collapse to proceed smoothly down the building.

Flash just right of northeast corner

 After this there is little obvious activity until the debris cloud reaches the level of the top of the pointed building in the foreground, when we begin to see more and more flashes in the lower portion of the building   These continue until the dust cloud reaches the bottom of the frame, with some bright enough to be seen through the growing dust cloud.

Flash on darker north side just below dust cloud

The location and timing of these flashes are consistent with the location of cutting charges in a controlled demolition,  though they no doubt represent only a small fraction of the number of charges that were involved.  Actually most of of the flashes would have been hidden from view, since those on the outer walls would have been placed on the inside of the perimeter columns, and those in the core would also be hidden .  

Similar flashes in WTC-1 collapse

A large number of small bright flashes can also be seen in this footage of the beginning of the north tower collapse.  In this case they seem to be going off in the debris cloud rather than in the intact building, as if these were "extra" charges that for some reason did not go off at the moment of demolition.

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