Editor’s note: This story contains graphic images. Viewer discretion is advised.

Gunfire rained down on country music fans as they attended a festival on the Las Vegas Strip on Sunday, and more than 50 people were killed in what is now the deadliest mass shooting in modern US history.

Photographer David Becker was covering the music festival for Getty Images when the shooting began late Sunday night. His photos from the scene show the frantic and bloody aftermath.

“People were fleeing, they were panicking,” he said. “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.”

For a while, Becker didn’t even know the popping sounds were gunfire. He thought at first they were firecrackers or bad speakers at the Jason Aldean concert.

“Then I started looking at my photographs, and what I was seeing was just unbelievable,” he said. “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.’ 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 realized people were dying.”

A man lays on top of a woman, shielding her from gunfire as others flee the festival grounds. Becker reported that they later got up and ran away.

Police take position outside the Mandalay Bay hotel, across the street from the concert venue. Police said the gunman fired on the crowd from the 32nd floor of the hotel.

People run from the festival grounds. “Once the crowd started to flee I went outside and saw a lot of people panicking and running for the exit that was right by the media tent,” Becker said. “I grabbed my camera and went back outside and found a place where I could see what was happening and also wouldn’t be in the way of people, so I stood on a table and started to shoot — thinking to myself still that this isn’t really happening, it’s just the speakers popping.”

People are seen on the ground after the gunman opened fire. “It was so dark I couldn’t really see what was happening,” Becker said. “There were a lot of people crying, speaking on cell phones and ducking for cover. As the crowd thinned out, I was able to go a little closer to try and see what was going on and take some more pictures.”

People gather around a victim outside the festival grounds.

Concertgoers help an injured person at the scene. “I’ve been doing this work for many years, and it is instinct to photograph first and ask questions later,” Becker said. “It probably seems irrational to just walk out and take pictures of people running for cover, but that is second nature for a photojournalist.”

A man makes a phone call as people run from the festival grounds.

Some people take cover at the festival.

A cowboy hat lies in the street after the shooting.

A man in a wheelchair is helped away from the scene.

The concertgoers were attending Route 91 Harvest, a three-day country music festival. Jason Aldean was performing when the gunfire began.

Broken windows are seen at the Mandalay Bay on Monday morning. The shooter was firing from the 32nd floor, police said. “When I stop and think, it’s hard to comprehend what I witnessed,” Becker said. “I was on autopilot, just doing my job capturing what was happening, which I think is important. Impactful images like these tell a story. They move people to think twice about doing anything like this, they move governments to change policies to prevent horrific acts like these happening again.”