“This is not an exaggeration: This shooting was going on for at least 10 minutes,” he added. “It was nonstop.”

Concertgoers described hearing round after round of gunfire. “Everyone was running, you could see people getting shot,” Gail Davis, one of the witnesses, said. “I’ve never been that scared in my life,” she added. “To have this happen, I can’t wrap my mind around it.”

“It just kept coming,” one of the witnesses, Robyn Webb, told The Las Vegas Review-Journal. “It was relentless.”

They said they saw about 20 people bleeding in the street.

“That’s when we knew for sure it was real,” said her companion, Matt Webb.

A photo posted by a Review-Journal photographer showed emergency responders transporting one injured person in a wheelbarrow.

As survivors poured into surrounding streets and buildings, and the police and paramedics streamed into the scene, unsure how many gunmen there were, the massacre shut down roads and highways; the police reported closing off about a mile of Las Vegas Boulevard and asked the public to steer clear of the area. Hours later, much of the city remained at shut down.

McCarran International Airport in Las Vegas said that some flights destined for the airport were diverted because of police activity. The airport is just east of the Mandalay hotel, and after the shooting there were reports of people fleeing the concert by running onto an airport runway.

The hotel itself was placed on virtual lockdown after the shooting, guests said.

“We went into the hotel and they started shutting down casinos,” Todd Price, a guest of Mandalay Bay, told CNN. “We tried to get into our rooms, and they shut down the elevators and started to get everybody out.”

The Route 91 Harvest Festival bills itself as “three days of country music on the Vegas Strip,” and Sunday night’s performance was the last of the festival. The site of the concert, the Las Vegas Village and Festival Grounds, run by MGM Resorts, sprawls over 15 acres and has a capacity of 40,000 people. The festival’s website said this year’s three-day concert was sold out.

In the first hours after the shooting, the police searched for a woman described as “a companion” of the gunman, Marilou Danley. Later, the sheriff said she had been located out of the country, and apparently was not with Mr. Paddock when he checked into the hotel, but that “he was utilizing some of her identification.”