A gunman opened fire from a hotel room above a packed, outdoor concert in Las Vegas, killing at least 50, wounding 400 more and prompting chaos as the panicked crowd scrambled for cover to avoid the onslaught from the deadliest mass shooting in modern U.S. history.

Clark County Sheriff Joe Lombardo said the shooter, identified as Stephen Paddock, 64, was found dead by officers who stormed his 32nd-floor room at the Mandalay Bay Resort and Casino.

Lombardo did not directly address the issue of terrorism but described Paddock as "a distraught person intent on causing mass casualties."

"We have no idea what his belief system was," Lombardo said. "We believe he was the sole aggressor."

Rita Katz, director of the SITE Intel Group that monitors terrorist groups, tweeted that the Islamic State issued statements saying Paddock was one of their soldiers and that he converted to Islam months ago.

The three-day Route 91 Harvest festival started Friday, and Lombardo estimated that 22,000 people were in the crowd when the shooting began Sunday at about 10 p.m. local time.

Country music singer Jason Aldean, who was on stage when the barrage of bullets began, continued to play through the first, extended volley, video from the scene shows. The crowd appeared confused, unsure what the sound was.

The music stopped at about the same time the volley did, but after several seconds another, lengthy round of gunfire began, setting off bedlam in the crowd. Videos on social media show people ducking for cover and fleeing amid the unrelenting bombardment.

“It was the craziest stuff I’ve ever seen in my entire life,” said concertgoer 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.”

Monique Dumas was six rows from the stage when she heard a burst of popping sounds she thought might be fireworks. She said she made her way out of the venue amid “organized chaos.”

“It took four to five minutes and all that time there was gunfire," she said.

Ivetta Saldana told The Las Vegas Review-Journal she hid in a sewer after the shooting began.

“It was was a horror show,” she said. “People were standing around, then they hit the floor.”

Lombardo, who said at least 10 guns were found in Paddock's hotel room, issued a plea for blood donations as local hospitals dealt with the wounded.

Aldean confirmed that he and the rest of his team were safe.

"Tonight has been beyond horrific," Aldean posted on Instagram. "I still dont know what to say but wanted to let everyone know that Me and my Crew are safe. My Thoughts and prayers go out to everyone involved tonight. It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night."

President Trump addressed the nation Monday from the White House, calling the attack "an act of pure evil." He thanked first-responders for their "miraculous" efforts and said he would visit Las Vegas on Wednesday.

Pope Francis expressed his “spiritual closeness” to victims and promised his prayers in a telegram addressed to the Bishop Joseph Anthony Pepe of Las Vegas.

Paddock lived in Mesquite, Nev., a city 80 miles northeast of Las Vegas. Lombardo said authorities had obtained a search warrant for the premises. Records show that Paddock resided at 1372 Babbling Brook Court in a home he purchased in 2015.

The house is in a retirement community, Sun City Mesquite, an upscale development of about 1,400 homes. The community includes an 18-hole golf course, indoor and outdoor swimming pools and a recreation center. Residents must be at least 55 years old, and no children are allowed.Lombardo said reports of multiple shooters and explosives were false. Some of the deceased were police officers, he added.

Lombardo said police had named Paddock's roommate, Marilou Danley, as a person of interest. She was located out of the country, and he indicated she had no involvement in the shooting. He said officers also located a Hyundai Tucson and a Chrysler Pacifica Touring, both with Nevada plates, which were registered to the dead suspect.

"Our condolences go out to the families who lost loved ones tonight. Thoughts and prayers are with all of those affected by this tragedy," the Las Vegas Metropolitan Police Department said on Twitter.

Nevada Gov. Brian Sandoval tweeted: "A tragic & heinous act of violence has shaken the #Nevada family. Our prayers are w/ the victims & all affected by this act of cowardice."

Contributing: The Associated Press