The following 45-second video of the demolition of Larry Silverstein's WTC 7 shows what appear to be flashes of explosions as the tower begins falling into its footprint. Claims are made that the video is fake, but a conclusive determination of its authenticity would require having access to the original video. It certainly looks real to me and it is exactly what an eyewitness told me he saw immediately before the collapse of the South Tower.
I spoke with an eyewitness whose office is near the World Trade Center. He had been standing among a crowd of people on Church Street, about two-and-a-half blocks from the South Tower, when he saw "a number of brief light sources being emitted from inside the building between floors 10 and 15." He saw about six of these brief flashes, accompanied by "a crackling sound" before the tower collapsed. Each tower had 47 central support columns.