go to pete ganci.e and deliver this message, the buildings are in danger of collapse. >> reporter: in this four-second video, at far left you see rich zarillo's blue shirt, he's delivering the warning to pete ganci. zarillo hardly got the words out when ganci's attention was drawn to a roar from the south tower above him. >> loud noise. had no idea what it was. all we saw was a plume of dust and smoke and debris. >> reporter: in the moment before, melissa doy had given the 9-1-1 operator her mother's phone number and the message that her daughter loved her. then there was silence. >> oh, my god. melissa, please. you'll be all right, you're going to be fine. you'll get to talk to your mother yourself. but you got to think positive, you got to stay calm. you're going to talk to your mother yourself, all right? melissa? >> reporter: palmer's last radio transmission was battalion 7 to ladder 15, and there's nothing after that. that's when the tower collapses. he must have known that with every step he asce