Transcript for What happened in Vegas mass shooting that killed at least 59
It was a horrifying case of deja Vu. We were about 15 feet away from the center stage to the left. Reporter: Once again people had gathered for music and fun, only to hear this. Instead. All of a sudden we hear the pop, pop, pop, pop, pop. We just thought it was fireworks. It kept going and going. It was insane. I could see the flashes from the mandalay bay. And I said I see where it's coming. Somebody shooting a fully automatic. People just started screaming and running. Reporter: At least 59 people killed. Now the deadliest mass shooting in modern American history. Among the victims, Lisa Romero, a high school secretary. Susan Smith, an elementary school office manager. Sandy Casey, a special Ed teacher. One off-duty officer was also killed. One of my officers was off duty attending the concert and lost his life. Reporter: More than 500 people were injured. The horror began nearly 24 hours ago when an estimated 22,000 concert Goers packed into an open-air venue along the famed Las Vegas strip for a country music festival. They had no idea that high above them on the 32nd floor of the mandalay bay resort, a lone gunman was watching and waiting. Shortly before 10:00 P.M. Local time, headliner Jason aldean took the stage. I was videotaping it. And all of a sudden hearing in the background something that sounded like fireworks going off. It was going pop, pop, pop. Reporter: At 10:08 P.M. The first shots erupted. All of a sudden you hear the music on the stage stop. Jason stopped singing. And then the lights on the stage went off. Oh, my god. Reporter: For so many confusion. Was this part of the show? That's just a firecracker. I know, but why would they do that? Reporter: Seconds later the rapid fire shooting starts again. I think someone said there's a shooter. He goes, that's gunshot. And we grabbed each other's hands and went running. We have an active shooter. Get down. We'll get trampled if we don't go. Reporter: The hellfire lasted for minutes coming from just across the street. I see the shots coming from mandalay bay, halfway up. A grandma and a little girl. And the little girl was just asking if she was going to die. She was scared. Reporter: Ambulances rushed to the scene as bystanders began to triage victims. We need to get people over to the hospital, okay? Okay. Go ahead. Put them all in the back. Put them all in the back. Reporter: Many plugging bullet wounds with their bare hands, turning barricades into makeshift stretchers. There's many people down. Stage left. Just be advised. Reporter: Hundreds of victims rushed to five area hospitals. Many with gunshot wounds. It was nonstop surgery after surgery until about 4:30, 5:00 this morning. Reporter: The state's only level one trauma center overwhelmed by the critically wounded. We brought in extra trauma surgeons, extra critical care nurses. Reporter: This doctor was the trauma surgeon on call last night. Tell me about the condition of the patients that were coming in. What were you seeing? We were seeing all kinds of injuries. Gunshot to the head, chest, abdomen, pelvis, extremities. Had is something we deal with on a regular basis. The only difference was the higher volume. Reporter: At 11:20 S.W.A.T. Teams are able to zero in on the room after the dense gun smoke sets off a fire alarm. Everyone in the hallway needs to move back. All units move back. Reporter: They use a flash bomb to enter the room. Breach, breach, breach. Reporter: They find him with a cache of 25 rifles and handguns. They identify the gunman as 64-year-old Stephen paddock from Mesquite, Nevada. They say he checked into the hotel on Thursday. His motive remains a mystery. We've checked all the federal databases and local databases and state databases and we had no knowledge of this individual. Reporter: Today my colleague Matt Gutman went into the mandalay bay. You can see the enormous distance between that shattered window where that shooter was perched across that park across the boulevard into that killing field below. What you see down there is a forensics team on the ground and those are people's belongings. Reporter: He caught up with Mike krunk who was in the thick of the rampage last night. My buddy was like I got hit. Reporter: Mike used his shirt to keep his friend from bleeding out. You are trying to do triage while the shooting was still going on. No way I would leave my buddy. We knew we were safe. Reporter: He eventually got him to a hospital. We need blood. If anybody can contribute blood. Reporter: Today those wanting to help heeded the urgent calls for blood donations. Some waiting in line up to five hours. This morning president trump addressed the nation ordering flags at half-staff, vowing to visit Vegas Wednesday. In moments of tragedy and horror, America comes together as one. Reporter: Later he and the first lady led the nation in a moment of silence. Many of the survivors now recounting their race to safety. Memories of other mass shootings on their minds. I was trying to be calm. But I thought -- at the moment I thought of the Orlando shooting. We didn't know where the shooter was. Reporter: Las Vegas now sits at the top of a list no city wants to lead. The worst mass shooting in U.S. History. The scenes all hauntingly familiar. From the pulse nightclub shooting in Orlando. Another mass shooting in America. Meese are describing this as a mass casualty situation. They said there's a lot of dead bodies in the club. They can't identify anybody. Reporter: To the massacre at sandy hook elementary school. From Newtown, Connecticut, where today a nightmare unfolded. Reporter: Modern mass murder in the U.S. Dates back to 1966 when a gunman opened fire from the bell tower at the university of Texas killing 14 people below. Since then the pace has quickened. The four deadliest shootings have all happened in the last decade. The pace and significance of mass shootings over the last decade in our country is saddening, how normalized it's become. Reporter: And many including survivors of previous attacks worry it will keep happening. It's haunted our dreams that we would wake up to the news of a massacre like this. This was domestic terrorism. Their hearts are broken. Reporter: Back in Las Vegas, hundreds gather for a prayer vigil tonight. Remembering the lives lost while honoring the heroes who saved so many others. And joining me now is Russell black and his fiancee Brianna Skogen. They were at the concert last night in the vip tent which was closest to the shooter. How did that unfold? Initially it started, we heard three, two, three round bursts, pop, pop, pop, pop, pop, pop. It sounded like an amplifier crackling. Reporter: You started videotaping. We were taking a photo together. We were going to get one last shot of the stage. And all of a sudden we heard that and I panned over and saw people going around the corner. I looked over, wait, that kind of sounded like an ar weapon. And then sure enough, they were fully automatic, which I was not ready for. I was expecting a few pops. I grew up in Kentucky around firearms my whole life. It just was relentless. We're talking hundreds of rounds. I mean, we couldn't move unless we waited for him to reload. Reporter: And Brianna, how did the crowd react? I mean, at that moment, I thought it was fireworks. And everybody was just kind of unsure what it was. Oh, maybe it was fireworks. Then pop, pop, pop, pop, pop, then everybody's eyes froze. I fell to the ground. People were like ducking for cover. People were getting shot at. Like you KO hear the ricochets around you. It was so scary. Reporter: You could see and hear the destruction. And you also saw real heroism, people stepping up into the breach. Absolutely. In a world where everyone's kneeling, those first responders stood up and charged headfirst into that, guns drawn, not knowing what they were going into. Knowing it was a fully automatic weapon. Everyone was crouched, hiding and stuff. I didn't see a single police officer take cover. Reporter: And you saw people administering first aid. Yeah. Thank god it was a country concert. A lot of exmilitary there. People of all ages just jumping into gear with whatever training they may have had. People putting fingers inside wounds to stop the bleeding, throwing fencing down to try to take multiple bodies out at a time. Reporter: You had time to reflect now. You said you've been emotional throughout the day thinking about it. Yeah. We both have. Just kind of comes in waves, huh? Just like when you start talking about it again, it just -- and you replay that sound or someone replays that video and you just hear the gunshots and hear the screaming and it just brings you right back to it. Then it's just like a whirlwinded of emotion. Reporter: Well, it's been a lot of trauma for you both. Thank you for tell us us what you saw. We'll go back to New York to
This transcript has been automatically generated and may not be 100% accurate.