good evening on this wednesday night from las vegas. and as we come on the air, we are following breaking news. abc news has learned of the gunman's extraordinary planning. possibly checking into multiple hotels before the attack, all within shooting distance of that concert site. just as we also see the police response through their body cams. the officers taking cover, then jumping into action. inside that hotel room, images of his deadly arsenal. 23 weapons. hundreds of rounds of ammunition. and we have new reporting tonight on the security guard, the first person to get to that hotel hallway, to the door of that hotel room. before the gunman shot him right through it. that guard, still able to tell police he'd found the gunman. abc's senior national correspondent matt gutman leading us off tonight. >> reporter: tonight, police body cameras reveal officers