More than 50 people were killed and 400 injured when a gunman opened fire at a country music festival in Las Vegas on Sunday, according to police.

The mass shooting — the deadliest in modern U.S. history — happened as country star Jason Aldean performed at the Route 91 Harvest Festival outside the Mandalay Bay Resort and Casino.

Video of the concert shows Aldean strumming his guitar as the sound of a semi-automatic weapon gunfire begins. Seconds later, Aldean is seen running from the stage to safety.

Eyewitness Meghan Kearney described the ensuing panic to MSNBC. “We heard what sounded like firecrackers going off,” she said. “Then all of a sudden we heard what sounded like a machine gun. People started screaming that they were hit… When we started running out there were probably a couple hundred [people] on the ground.”

She continued: “People kept dropping and dropping… People were getting shot one foot away from us. People were trying to save their friends. There were gunshots everywhere. Helping them would’ve meant that we got shot too.”

Two on-duty officers were injured in the shooting. One was in stable condition after surgery and the other sustained minor injuries, according to authorities.

SWAT teams were sent to the scene at 10:08 p.m. local time. Nearly two hours later, police on Twitter indicated that “one suspect is down.” Officials said the shooter was “lone wolf” Stephen Paddock, 64, of Mesquite, Nevada, NBC News reported. He died from a self-inflicted gunshot wound. Additional weapons were found in his hotel room, according to authorities. Officials were initially also looking for a woman, 62-year-old Marilou Danley, who was described as Paddock’s roommate in Mesquite. She was located out of the country and authorities said she was not involved with the shooting, ABC reported.

“Tonight has been beyond horrific,” Aldean wrote 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. #heartbroken #stopthehate.”

Reported Shooting At Mandalay Bay In Las Vegas
Credit: David Becker/Getty Images
Reported Shooting At Mandalay Bay In Las Vegas
Credit: Ethan Miller/Getty Images

“Our thoughts & prayers are with the victims of last night’s tragic events,” the Mandalay Bay Resort said in a statement. “We’re grateful for the immediate actions of our first responders.”