Let's ask some chiefs
"We were there, I don't know, maybe 10, 15 minutes and then I just remember there was just an explosion. It seemed like on television when they blow up these buildings. It seemed like it was going all the way around like a belt, all these explosions."
-Fire Battalion Chief Dennis Kenahan
“Somewhere around the middle of the World Trade Center, there was this orange and red flash coming out. Initially it was just one flash. Then this flash just kept popping all the way around the building and that building had started to explode. The popping sound, and with each popping sound it was initially an orange and then red flash came out of the building and then it would just go all around the building on both sides as far as I could see. These popping sounds and the explosions were getting bigger, going both up and down and then all around the building."
-FDNY Captain Karin Deshore
“I was ... hearing a noise and looking up.... The lowest floor of fire in the south tower actually looked like someone had planted explosives around it because ... everything blew out on the one floor. I thought, geez, this looks like an explosion up there, it blew out.”
-Battalion Chief Brian Dixon
"While I was still in that immediate area, the south tower, 2 World Trade Center, there was what appeared to be at first an explosion. It appeared at the very top, simultaneously from all four sides, materials shot out horizontally. And then there seemed to be a momentary delay before you could see the beginning of the collapse."
-FDNY Chief Frank Cruthers
"Then we heard a loud explosion or what sounded like a loud explosion and looked up and saw Tower to start coming down. Crazy."
-Fire Battalion Chief John Sudnik