Las Vegas shooting: Eyewitnesses describe hearing 'pop, pop, pop' at festival

— -- Members of the production company 3G, which put on the music festival where a gunman opened fire Sunday night, described for ABC News Monday the moment they knew the "pop, pop, pop" they were hearing was actually gunfire.

Keith Conrad and his wife, Amy Conrad, told "World News Tonight" anchor David Muir that they were on the far southwest side of the Route 91 Harvest Music Festival in Las Vegas, Nevada, closest to Mandalay Bay Resort and Casino, getting ready to go behind the stage.

"We kind of heard a 'pop, pop, pop,' and one person said, 'It's maybe a firecracker,'" said Keith Conrad, COO of 3G. "We actually saw the light up in the Mandalay Bay tower. And we thought the second time it [gun shots] went through, it really was gunfire. And I said, 'Get down!'"

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At least 59 people were killed and at least 527 were injured Sunday night, when a gunman opened fire on the festival from the 32nd floor of the Mandalay Bay Resort and Casino. It was the worst mass shooting in modern U.S. history.

Among the dead: a registered nurse from Tennessee, a school worker in New Mexico and at least six victims from California. Police identified the shooter as Stephen Paddock, 64, of Mesquite, Nevada. Police said Paddock later killed himself inside a Mandalay hotel room.

In a statement, Keith Conrad said the company had approximately 100 employees at the festival. He said that one of its team members had been shot multiple times but was now stable.

"The rest of our 3G family has been accounted for," he said in the statement. "There are just no words to express the devastation of what we witnessed last night, and our thoughts and prayers are with the many people who have been impacted by this terrible, senseless tragedy."

Amy Conrad told Muir on Monday that the most terrifying part of the night was hearing the gunfire end, thinking it was safe to stand up and then hearing the gunfire begin again.

Keith Conrad said his father-in-law, the president of the company who had also attended the festival, had likely been saved by a backpack he'd been wearing Sunday night. Keith Conrad said his father-in-law had discovered his camera, which was in the backpack, completed destroyed today.

"[It] absolutely saved his life," Keith Conrad said of the backpack.

"We were lucky," Amy Conrad said.

Two sources familiar with the investigation said there was a combination of rifles and handguns at the scene. The motive for the attack was unclear.