Firefighter Daniel Rivera:
"At first I thought it was... Do you ever see professional demolition where they set the charges on certain floors and then you hear, 'pop-pop-pop-pop-pop'? That's exactly what, because I thought it was that, when I heard that frigging noise, that's when I saw the building coming down."
 
NYFD Captain Karin Deshore:
"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 kept popping, all the way around the building, and that building had started to explode." 

156 Eyewitness Accounts of Explosions:
http://www1.ae911truth.org/downloads/156eyewitnessaccounts.pdf
https://www.youtube.com/watch?v=cZ4dVo5QgYg