Recording artist Jason Aldean performs during the Route 91 Harvest country music festival at the Las Vegas Village on October 1, 2017 in Las Vegas, Nevada.

Country star Jason Aldean was singing on stage at the sold-out Route 91 Harvest festival on the Las Vegas Strip when a burst of semi-automatic gunfire erupted.

Not realizing what was happening, he tipped his hat and continued singing. Moments later, he ran off the stage, still holding his guitar, according to videos posted on social media.

"It was a horror show," Concertgoer Ivetta Salda the Las Vegas Review-Journal. "People were standing around, then they hit the floor." She said she hid in a sewer.

The gunfire rang out from an upper-level floor of the Mandalay Bay hotel, which is across the street from the festival, a sold-out three-day gathering.

Police said at least 58 people were killed and more than 500 were injured. The death count made it the deadliest mass shooting in modern U.S. history. Until Sunday, the deadliest recent shooting was the June 2016 rampage at the Pulse nightclub in Orlando, where 49 people died.

Some of the wounded in Nevada were carried away in wheelbarrows and luggage carts, according to the Las Vegas Review-Journal.

The gunman, Stephen Paddock, 64, of Mesquite, Nevada, was found dead on the 32nd floor of the Mandalay Bay in an apparent suicide, authorities said. Paddock had no known connection to terrorism, according to investigators.

"My thoughts and prayers go out to everyone involved tonight," Aldean wrote on Instagram. "It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night."