I've read and viewed so much material about the events of 9/11 that I'm rather surprised to have missed this.
It's a video called 911 Eyewitness 1 of 3. It appears to be the source footage of the short clips posted in another thread, which appeared to show an explosion (evinced by a camera shake) followed by a debris cloud emanating from the base of the towers. This fuller video shows more of the same and is worth watching for that alone. However, for those who run out of patience, skip to the segment that begins at 19:08. Watch the helicopter that is circling the tops of the towers. At 19:17 the first in a sequence of flashes appears to come from the helicopter. The footage is slowed down and shown again, starting at around 22:20.
Can anyone explain what these flashes might be?