. >> reporter: sergeant andy brennan, trained hostage negotiator, was working an off-duty detail at a bar downtown that night. when he responded to the signal 43 call at pulse. and headed to the dispatch center. >> so i went and sure enough, five minutes later, this shooter calls in. >> where are you at? >> in orlando. >> where in orlando? >> reporter: the shooter hung up, as he would again and again that night. >> chilling. i said, let's call. so we made our first call. immediately. from that dispatcher's station. >> hello there. hi, there. this is orlando police, who am i speaking with, please? >> you're speaking with the person who pledges allegiance to the islamic state of abu bakr al baghdadi -- >> we wanted to confirm we had in fact the person that was inside. you can start to hear the serious tone in his voice. >> reporter: for the next 55 minutes, brennan will engage the shooter on the phone, listening to his rants about isis and syria. >> they need to stop the u.s. air strikes. you have to tell the u.s. government to stop bombing. >> i understand that. but here's why i'm here