The photographer behind some of the most haunting photos from the Las Vegas mass shooting has given his account of what happened.

Getty photographer David Becker had been filing photographs of the Route 91 Harvest country music festival when he heard “very loud popping sounds,” he told the Washington Post.

“I went outside to see what was happening and a security guy said it was just firecrackers, so I went back to work. The second time I heard the popping sounds, somebody said to me, “It was just speakers or sound equipment,” and again, I went back into the media tent. Then the noises went again, and that was when the crowd started to flee.”

He grabbed his camera and headed back outside, finding a place where he wouldn’t be in the way of people. It was so dark that he couldn’t really see what was happening, still thinking to himself that it was just the speakers popping.

“There were groups of people helping each other everywhere and a real sense of people running for cover. People were fleeing, they were panicking. The gunfire was sporadic, it would stop and then more shots, then a lull and then more shots. I could hear people yelling at them to shut off the lights, to be quiet. People were cowering — they were very fearful for their lives. A woman tripped right in front of me, a man shielded a woman with his body before I saw them both get up and run away, a man in a wheelchair was helped to an exit. I was trying to capture anything that was moving and that had good lighting. That was critical, it was so dark and there was limited lighting.  It was really hard to get a sense of what was happening. At this stage I still just thought it was a speaker popping, so I was trying to capture people’s emotions and a sense of the panic that was around me.”

He said it was only when he returned to his tent did he hear words like “Code Red” and “police perimeter”, and started to understand what was happening.

“It was then I started looking at my photographs and what I was seeing was just unbelievable. It had been so dark outside I couldn’t see the details. I just saw a lot of people laying on the ground thinking they were playing possum, but now I could see people covered in blood and I thought, this is real.”

One of the women he captured had blood on the back of her legs, and he says he couldn’t be certain if she was alive.

“When I saw the image of the woman lying on the ground covered in blood, that was when the impact of what I was experiencing hit — when I realised people were dying,” he said.

Becker told TIME Magazine that the couple in the photograph lead photograph got up moments later and ran.

“I don’t know if she was injured, but he was very clearly shielding her with his body and protecting her,” he said.

He added that while he understood that it probably seemed irrational to walk out into a chaotic situation, it’s first instinct for a photojournalist to “photograph first and ask questions later.”

At least 59 people lost their lives and another 527 people were injured after gunman Stephen Paddock opened fire on the crowd.

It’s the deadliest mass shooting in modern American history, prompting renewed calls for stricter gun control across the United States.

Image: [All] Getty Images / David Becker