Transcript for New details about Las Vegas shooter emerge
out of the senseless violence. Andrea Castilla was out celebrating her 28th birthday with her sister Athena and boyfriend Derek Miller who had been waiting for the right moment to propose. I told him, she would have said yes immediately. Reporter: The country music fans were having the time of their lives, when terror struck. My sister was like, duck! So we duck. Next thing you know, she was already hit. It happened so fast -- It happened so fast, just happened so fast. Reporter: Andrea shot in the left side of her head. Oh, so much blood, I've never seen so much blood in my entire life. Reporter: She fell to the ground. Her sister Athena shielding her from incoming fire. Shielding her not just from bullets but -- From the panic, because people were trampling, running through. And I just -- we were just trying to shield her, you know. Reporter: Using a gate as a makeshift stretcher, her sister finally gets her to a pickup truck, taking victims to the hospital. Couldn't give up on her, she deserved a chance to get help. She was literally dying in my arms. What did you say to her in those moments? Just, you're our angel, you're going to be fine, don't leave me. Reporter: With so many victims and Andrea without her I.D. The hospital mistakenly told the family she was still alive. For nearly seven hours she held out hope. I was trying to stay positive that she was going to make it. But I could see all the blood, it was all over me. Essentially, she died in your arms. Yeah. Reporter: After their mother had died of cancer, Andrea wanted to use her talents as a makeup artist to make cancer patients feel beautiful. She was a beautiful soul. There's not much more to put it. She shouldn't have died that night. They lost one of the most beautiful souls on this planet. Every single room she walked into, she lit up. Reporter: For Jenna decandio, it was a delayed graduation present. A big country western fan? We all are, we go to concerts together. Reporter: The tight-knit trio, Jenna and her parents, Jill and George, driving from North Carolina to see Eric church, Jenna's favorite. On the last night, terror struck. We just saw Jason aldean, all the people on the stage just drop everything and run. Reporter: Already hobbled with a broken foot, Jen ka leaping on her parents for support feels a burning sensation. I was shot. And they took at me, what? My mom sees the blood is just like coming out. She found something and pushed it on my hip. There were dead bodies in the street. And that's when it hit me. Not only are we running, we are really running. How were you able to hold it together? It's incredible. I knew that if we stopped, the chances of us getting out weren't very good. You knew you were doing this for your parents. I knew we had to keep going. Reporter: Now she's here, recovering after surgery. The bullets still lodged in her body, a reminder of what could have been. It makes you stop and appreciate what you have. And I'm really fortunate to have both of my parents still. She saved all of us. I mean, she's a hero in my book. Reporter: Today the president and first lady traveled to las Vegas, meeting with victims, their families, and first responders. We know that your sorrow feels endless. We Stant together to help you carry your pain. You're not alone. Reporter: Tonight the las Vegas sheriff providing new details about the the shooter. He was doing everything possible to figure out how he could escape at that point. Reporter: And they say they're actively looking for possible accomplices. It's troublesome that this individual was able to move this amount of gear into a hotel room unassisted. Reporter: Today authorities questioning the woman who may have known the shooter best, his long-time girlfriend, Mary Lou danly, who was in the Philippines at the time. She arrived back in the U.S. This morning telling the FBI, according to law enforcement, she had no knowledge of his plans or that he had bought so many guns. Her lawyer read a statement on her behalf. He never said anything to me or took any action that I was aware of, that I understood in any way to be a warning that something horrible like this was going to happen. Reporter: According to Danley's sister in Australia, who wanted their faces blurred, paddock sent her to the Philippines just two weeks before the shooting. Sent her away. So that he can plan what he's planning without interruptions. Reporter: Police say he was gambling right before the massacre. Stephen paddock is a man who spent decades acquiring weapons and ammo and living a secret life. Much of which will never be fully understood. Reporter: Today a clearer picture of how the shooter meticulously planned his attack. Federal sources say he looked into a rented room in this building, right next to another music festival featuring Lorde, the week before. Inside the mandalay bay Sunday night, the shooter installing his own surveillance system. A camera hidden on this room service cart. Another behind the peephole looking down the hall. All rigged so he could see police approaching. Investigators have recovered 23 firearms inside the hotel. 12 of them rifles. Each modified with a bump stock that enabled them to fire like machine guns. An onslaught of firepower that took a devastating number of lives. Earlier this week a candlelight vigil for police officer Charleston Hartfield, a member of the Nevada army national Guard. And a youth football coach. Even though officer Hartfield was at the concert as a civilian, he immediately took action to save lives. In that moment, he was acting as a police officer. He ultimately gave his life protecting others. Reporter: Sonny Melton, 29-year-old registered nurse from Tennessee, saving his wife's life before taking a bullet in the back. He wrapped his arms around me from behind and we started running. And that's when I felt him get shot in the back. He was still breathing at that time. But then he stopped. Reporter: 5-year-old Aidan Hoosier got separated from his mom, aunt, and sister. I didn't know where to run. Reporter: Luckily Lindsay Rogers, a complete stranger, grabbed him, posted his picture on social media, and got him home. He was really brave and really good. He didn't cry or anything. Reporter: The horror is still fresh. Today we were talking about post-traumatic stress syndrome and things like that. The counselor was saying, this is what you might expect, how are you doing? She says, I've had nightmares. We have to remember and remind ourselves, this is absolutely abnormal, this is absolutely not okay. Reporter:ed this night therapist Dr. Michelle Paul rushed to the scene to help victims suffering from the shock and horror that she says could leave emotional scars for years to come. The nervous system has just been under assault. And it's going to respond by shaking, by uncontrollable tears apparently coming out of nowhere. When someone experiences a trauma like that, as the weeks progress, how do you know that something's wrong? After about a month, if that's still happening, it's time to definitely reach out to a mental health professional. Reporter: Andrea Castilla's family says Athena and Andrea were more than sisters, they were best friends. What do you want the world to know about Andrea, your sister? That she was a strong soul, and to anyone else that's suffering from loss from this horrible night, I mean, hang in there and be strong. How about you, what will you miss? Kind of miss her smile, she brought so much joy and happiness. Reporter: Their bond almost perfectly immortalized by none other than Jason aldean. ??? Oh man we were living didn't
This transcript has been automatically generated and may not be 100% accurate.