A gunman on a high floor of a Las Vegas hotel rained a rapid fire barrage on an outdoor concert festival on Sunday night, leaving at least 59 people dead, injuring 527 others, and sending thousands of terrified survivors fleeing for cover, in one of the deadliest mass shootings in American history. 

Online video of the attack near the Mandalay Bay Resort and Casino showed the singer Jason Aldean’s performance at the Route 91 Harvest Festival, a three-day country music event, being interrupted by the sound of gunfire. 

The music stopped, and as victims fell bleeding, concertgoers screamed, ducked for cover, or ran. At least 23 firearms, including a handgun, were found in Paddock’s hotel suite, according to the Las Vegas Metropolitan Police Department. 

The police found the gunman, whom they identified as Stephen Paddock, 64, dead in his room at the hotel. Investigators were still combing through Mr. Paddock’s background and searching his home on Monday. 

The Islamic State claimed that Paddock was one of its soldiers, but did not provide any evidence of its claim. The F.B.I. said there was no evidence so far that Paddock had ties to any international terrorist organization, and relatives said he had not displayed strong political or ideological beliefs in their interactions with him. 

Speaking at the White House, President Trump condemned the shooting as an “act of pure evil” and called for the country to come together, saying, “Our unity cannot be shattered by evil, our bonds cannot be broken by violence.”