Las Vegas shooting: What photographer backstage during Jason Aldean's performance saw when gunfire started

— -- Photographer Brandon O'Neal was backstage during country star Jason Aldean's performance at the Route 91 Harvest Music Festival on the Las Vegas strip Sunday night, when he suddenly heard a series of loud pops.

At first, O’Neal said he thought it was speakers on stage popping. But then the sound happened again. And then a third time.

“It was so loud I put my hand over my ears and was like, ‘Oh my god, the speakers are about to blow up,’” he told ABC News’ Juju Chang in an interview for “Nightline.” “And then everything just-- everything stopped.”

The sound he was hearing was gunfire from a shooter, identified by police as 64-year-old Mesquite, Nevada resident Stephen Paddock, who had opened fire on 22,000 concertgoers, sending them running for their lives. The attack killed at least 59 people and injured 489. Paddock was found dead on the 32nd floor of the Mandalay Bay Resort and Casino when authorities entered his room.

The concert was being held across the street from the Mandalay Bay hotel, and O’Neal said it became total panic and confusion as the sound of rapid gunfire echoed throughout the venue.

“We had no idea if there were people in the venue with guns or if they were from the hotel,” he said. “No one knew where it was coming from...We had no idea where to go.”

O’Neal kept his camera rolling as he was running for his life. Footage he gave to ABC News shows the horror of the gunfire. When he later learned the gunfire was coming from a window at the hotel, he said it was like looking at “the devil's lair.”

“We ran around the stage and we could hear bullets like whizzing by like—Ricocheting,” he said. “That's when we made it to this exit to try to go to the parking lot and there was a cop car there so we went behind the cop car.”

In the chaos, O’Neal said a girl next to him was shot.

“My friend and a couple of other people picked her up and brought her over to the police vehicle to just get her out of harm's way,” he said. “She just she wasn't moving. I don't think she made [it].”

He said he saw people running back into fray to try to save others. Everywhere he looked, O’Neal said, he saw people covered in blood.

“You didn't know if they were shot or not,” he said. “I saw a bunch of people just laid out on the floor. I don't really want to describe what I saw. It's just -- not good.”

O’Neal and his friend made it out of the venue physically unharmed, but emotionally, he said he is still dealing with the horror of what he went through.

“When I wake up, [I’m] praying it was a nightmare,” he said. “And then it’s real.”

ABC News' John Kapetaneas contributed to this report