It's falling debris catching the light. It's quite clear in the .mov I have, which may be a higher quality vid. There was lots of falling debris, some reflective, some not. There were flashes all up and down that side of the building, the sunny side. The flashes had a pattern, from high to low, in rythm, at a certain speed, all of which match perfectly my sense of a piece of glass falling, rotating, flipping, and catching the sun when the rotation was correct. It's almost obvious that's what it is in the .mov. One piece even angles over to the edge of the building and glints once right on the junction between the building and the smoke cloud.
Get the .mov, that's all I CAN SAY BECAUSE IT WILL LEAVE NO DOUBT.