A gunman firing from a Las Vegas hotel rained a rapid-fire barrage on a huge outdoor concert festival on Sunday night, sending thousands of people fleeing until SWAT units found and killed him. More than 50 victims died, and at least 200 others were wounded, officials said, making it one of the deadliest mass shootings in United States history.

Online video of the attack outside the Mandalay Bay Resort and Casino showed the country singer Jason Aldean performing outside at the Route 91 Harvest Festival, a three-day country music event, interrupted by the sound of automatic gunfire. The music stopped, and concertgoers ducked for cover. “Get down,” one shouted. “Stay down,” screamed another.