Happens in the first minute or so. Google "Windsor Tower."
Could transformers or other electrical equipment explain some of what the firemen saw and heard? What about an acre of concrete floor slamming into another? Would steel bolts snapping under tremendous tension make a pop or explosive sound? Assuming the towers weren't in the vacuum of space, we can be fairly safe to say the things I mentioned are good candidates to explain what the firemen heard. Even they think so...