Roy Chelsen

Many witnesses hear explosions during the collapse of the north WTC tower. Some report hearing a single explosion:
Reporter Mike Sheehan hears “another deafening explosion. I looked up and saw the top of the North Tower, the mast, begin to fall.” [Gilbert et al., 2002, pp. 126]
Fire Lieutenant William Wall: “[W]e heard an explosion. We looked up and the building was coming down right on top of us.” [City of New York, 12/10/2001]
Firefighter Roy Chelsen: “All of a sudden we heard this huge explosion, and that’s when the tower started coming down.” [City of New York, 1/18/2002]
EMT Jason Charles: “I heard a ground level explosion and I’m like holy shit, and then you heard that twisting metal wreckage again.” [City of New York, 1/23/2002]
Firefighter Kevin Murray: “When the tower started—there was a big explosion that I heard and someone screamed that it was coming down.” [City of New York, 10/9/2001]
Firefighter James Ippolito: “I heard an explosion and turned around and the building was coming down.” [City of New York, 12/13/2001]
Fire Lieutenant Gregg Hansson: “[A] large explosion took place. In my estimation that was the tower coming down, but at that time I did not know what that was. I thought some type of bomb had gone off.” [City of New York, 10/9/2001]
Firefighter Kevin Gorman: “I heard the explosion, looked up, and saw like three floors explode, saw the antenna coming down.” [City of New York, 1/9/2002]
Others report hearing multiple explosions:
EMT Gregg Brady: “I heard 3 loud explosions. I look up and the North Tower is coming down now.” [City of New York, 11/1/2001]
Firefighter Richard Carletti: “I remember seeing the antenna do a little rock back and forth and I could just hear the floors pancaking. I heard it for about 30 pancakes, just boom, boom, boom, boom.” [City of New York, 1/2/2002]
Fire Lieutenant Michael Cahill: “That’s when the second collapse started to come down. All kinds of noise. Boom, boom, boom, boom, boom, very loud.” [City of New York, 10/17/2001]
Firefighter Sal D’Agostino is actually inside the North Tower, around its fourth floor, when the collapse occurs. He says, “It’s pancaking from the top down, and there were these huge explosions—I mean huge, gigantic explosions.” [Providence Journal, 9/11/2002; Dwyer and Flynn, 2005, pp. 241-242]
Firefighter Bill Butler, who is with D’Agostino inside the tower, says, “It was like a train going two inches away from your head: bang-bang, bang-bang, bang-bang.” [Providence Journal, 9/11/2002]
EMT David Timothy: “[Y]ou started hearing more explosions. I guess this is when the second tower started coming down.” [City of New York, 10/25/2001]
CTV will later assert, “When eyewitnesses claim to have heard explosions prior to the collapse, those were just the sounds of a massive building contorting and crushing anything inside.” [CTV, 9/12/2006]