People walk shortly after sunrise Monday near the Las Vegas Strip, where the deadliest mass shooting in modern U.S. history occurred Sunday at a music festival.

LAS VEGAS — The rapid-fire popping sounded like firecrackers at first, and many in the crowd of 22,000 country music fans didn’t understand what was happening when the band stopped playing and singer Jason Aldean hustled off stage.

“That’s gunshots,” a man could be heard saying emphatically on a cellphone video in the nearly half-minute of silence and confusion that followed. A woman pleaded with others: “Get down! Get down! Stay down!”

Then the pop-pop-pop noise resumed. And pure terror set in.

“People start screaming and yelling and we start running,” said Andrew Akiyoshi, who provided the cellphone video to The Associated Press. “You could feel the panic. You could feel like the bullets were flying above us. Everybody’s ducking down, running low to the ground.”

While some concertgoers hit the ground Sunday night, others pushed for the crowded exits, shoving through narrow gates and climbing over fences as 40- to 50-round bursts of automatic weapons fire rained down on them from the 32nd floor of the Mandalay Bay casino hotel.

By Monday afternoon, 59 victims were dead and 527 injured in the deadliest mass shooting in modern U.S. history.

“You just didn’t know what to do,” Akiyoshi said. “Your heart is racing and you’re thinking, ‘I’m going to die.”’

The gunman, identified as Stephen Craig Paddock, a 64-year-old retired accountant from Mesquite, Nev., killed himself before officers stormed Room 135 in the gold-colored glass skyscraper.

The avid gambler, who according to his brother made a small fortune investing in real estate, had been staying there since Thursday and had busted out windows to create his sniper’s perch roughly 500 yards from the concert grounds.

The motive for the attack remained a mystery, with Sheriff Joseph Lombardo saying: “I can’t get into the mind of a psychopath at this point.”

With hospitals jammed with victims, authorities put out a call for blood donations and set up a hotline to report missing people and speed the identification of the dead and wounded. They also opened a “family reunification center” for people to find loved ones.

More than 12 hours after the massacre, bodies covered in white sheets were still being removed.

The shooting began at 10:07 p.m., and the gunman appeared to fire unhindered for more than 10 minutes, according to radio traffic. Police frantically tried to locate him and determine whether the gunfire was coming from Mandalay Bay or the neighboring Luxor hotel.

“It was the craziest stuff I’ve ever seen in my entire life,” said Kodiak Yazzie, 36. “You could hear that the noise was coming from west of us, from Mandalay Bay. You could see a flash, flash, flash, flash.”

The crowd, funneled tightly into a wide-open space, had little cover and no easy way to escape. Victims fell to the ground, while others fled in panic. Some hid behind concession stands or crawled under parked cars.

Couples held hands as they ran. Some of the bleeding were carried out by fellow concertgoers. While dozens of ambulances took away the wounded, some people loaded victims into their cars and drove them to the hospital.

Some of the injured were hit by shrapnel. Others were trampled or were injured jumping fences.