Transcript for Bodycam video shows moment police breached Las Vegas shooter's hotel room: Part 1
These hallowed ground. Until the tragic story. We're one of uncommon hill elusive. And Tarrant County yeah believe it's only visions of balloon. Hundreds of people running everywhere yes. Here this is gonna just be the first part of a multifaceted attack over the last year's much of that night has remained a mystery trying to get her. The first and only news will be some captured on cell phone. But after a ten month long investigation by law enforcement in coordination with MGM using surveillance cameras go. I'm cameras and. And a 3-D model. Yeah. We can now present and most comprehensive use of mass shooting. An explosion gaps in the system and showing the response at the moment yeah. September 25 2017. Security camera footage captures Stevens had a arriving at the Mandalay bay resort and casino in six days before the attack and he's a high roller a regular at the hotel came. Was a very typical gas he was in our estimation. Those lowest risk type of individual L alarm bells going off. Katic checks into a suite on the 32 floor and he's given the VIP treatment allowed to bring his luggage up through service elevator. Over the course of his stay he makes several trips to his house in mesquite Nevada and brings in case after case after case of non cash. 21 suitcases in all full of guns and ammunition. The investigation shows on October 1 T orders room service and rich surveillance cameras and before he brings in a final batch of suitcases. And locks himself in his room. That night at 9:40 PM Mandalay bay security officer Hayes whose campus is on a routine security check assigned to a room alarm on the 32 floor. This was the last column I'm. Your head and how many ahead and I had around us. Home Frey after this. He has no idea he's about to become an accidental hero on what would become one the most horrific nights in American history. As shown in this rendering provided exclusively to ABC news by MGM. Once on the thirtieth floor he takes the stairs to the 32. But he finds the exit door jammed. So we walks up to the elevator and takes a back down. When did. Something seems off to you when I noticed narrow L bracket those secured that hold the door secured. That bracket strategically placed there by men staying in the sweet just a few feet away I didn't know I was going on just simply because that's not normal. I had to call are sicker dispatch. I was transferred two engineering dispense. As he walks back into the hallway to check that room alarm turns out a nanny a few doors down lift their door ajar. Here's a strange noise coming from this week's photos drill noises. The massacre has just begun. Across the just moments before. Jason no team had taken the stage in the final night of the route 91 harvest festival. Let's read them in the the new thing that's. Confusion makes his. I'm doing. Concert goers plugged timeline. Yeah hammer and her heart yeah very critical helping get here. Yeah. That comes through the second floor Campos who is unarmed continues walking down this hallway. He passes a room service cart with those surveillance cameras he cheated out of the sound of the stairway doors that alerts the shooter he fires through the do. Yeah. Suddenly you're under fire yes went to go lift up my pat and I and I saw blood coming down. Campos takes cover in this doorway alcove. And radios for help. Those. Yeah. One big fire. The shooter turns back to the concert goers below and fires over 250. More rounds. In the next few. They just mentioned police officers respond to this he and brought. The. But did you come under tough. Pinned down by automatic gunfire. The echo of gunfire through the stripped leads to mistaken reports of shooters and multiple hotels. Deputy chief Andrew Walsh response to the scene but success. Me is that fear that pain in my stomach that this was gonna just be the first part of a multifaceted attack. We report well part of Newt you're just prior down. Tropicana. Back in the hotel Stephen shock an engineer is riding an elevator to the 32 floor. In response to campuses earlier call about that L Brackett. Pushing his maintenance cart he walks out of the elevator. And street into danger. And noticed him I said get cover it's not safe at that moment in time there was more rounds being dispersed. Suddenly show can self is under fire some hit me in the back so is jumping in the cover. What we're glad I got up vehicles fired a rifle on the thirty. The Mandalay bay security office here's shocks alert and hustles toward the elevators. I thought. Follow come out of this hole in the lives. I want to communicate for metro and responders to get up there this is where the shooters. Every second that shocking campus are under fire on the 32 floor. He's a chance for the concert goers who moved to clean. For page no Lance and running with an agonizing choice. She and her mother were both shot. We brought her over. And a retired firefighter came over and. It who suddenly the goal. If there but we wanted to land that we needed to go I can't do if I could. Beneath that you. You need to go. Page and her sister fled leaving their mom Rose Marie behind in the care of a stranger. Montana shut eye shut down session to name page or ran over me same mom just screaming in my name mom and that's and then my body just floated. Floated up. The evidence plaque. Thank you see everybody. It has in mind my own body laying down there. The next thing I know hasn't happened. And I saw my dad and nine teen and others. My uncle. It was so beautiful it was so obviously wanted to combat that is told me it's not your timers marine compacts it's time. So next thing I woke up. And a key city of me on that dragged me to ambulance that retired firefighter kept his word getting her to safety. And hundreds fled into the nearby streets and. That would be set up makeshift triage and it's. Yeah. It's. Nice getting and it becomes situations and dying here police officers sticking his paramedics and see officers stopping. To render aid UC office. Bleeding from bullet wounds in the amateur video quit their and I couldn't got a gunshot their lungs I don't know yeah. In the midst of the chaos. Police worked to sleep every when com. They apparently have been there yet it will be back. Steve business.
This transcript has been automatically generated and may not be 100% accurate.