Transcript for Las Vegas shooter checked into multiple hotels before the attack

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 risking their own lives amid unceasing gunfire. Get down! Go that way! Get out of here, there's gunshots coming from over there. Go that way! Reporter: With sirens blaring and bullets flying, officers huddled behind a wall, trying to pinpoint the source of the mayhem. North of the mandalay bay. Coming out of a window. Reporter: Exhibiting extraordinary calm, those officers decting concert-goers away from the shooting to safety. This way, this way, this way. Go, go, go. Reporter: It would be an hour before police breached Stephen paddock's luxury suite. Tonight, these photos revealing their destructive power. This assault Reichel, one of 12 outfitted with an attachment called a bump stock. This video common straiting how it works. On the tip of the rifle, a flash hider, making its fiery blast difficult to spot. And in the corner, a pile of 17 magazines holding 100 rounds each. You're looking at 1,700 bullets. Pad dom dock installing his own surveillance camera. All rigged so he could see police approaching. And now, we are learning he spotted a hotel security guard. He was advancing towards the room when the suspect fired through the door at the security guard and struck him. Reporter: Tonight, that security guard identified as Jesus campos. Somehow he was able to get away and tell the tactical team that pad dom was in room 135. An hour later, they breached the door. Breach, breach, breach. Reporter: And discovered him already dead. Tonight, federal law enforcement sources telling ABC news they are investigating reports that the day before the massacre, he checked into multiple hotels overlooking the concert venue. And he may have looked for other opportunities. Federal sources say he looked into renting a room in this building right next to another music festival just the week before. And from this perch he would have had a clear field of fire right down into those tens of thousands of people packed into that concert below. But for some reason, he didn't pursue the plan, instead, he waited a week for the harvest festival. And Matt Gutman joins us live tonight here in Las Vegas. Matt, we now know he may have had several hotel options for this attack, ument matly spending more than three days in the mandalay bay hotel behind me. And you're learning what he might have done the day before? Reporter: That's right, David. We're learning he wasn't just shuttered up in that luxury suite in that hotel. Now, law enforcement hasn't weighed in on this yet. I spoke to a casino with worker who said they saw him play high stakes video poker for eight continuous hours the morning of the attack. 16 hours that weekend. And what takes many so disturbing is that it means he was milling about with the public, out in the open, for days before the attack. David? Matt Gutman leading us off tonight. Matt, thank you.

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