UPDATE (10/2/17, 12:15 p.m. ET): The death toll has risen to 58, and the number of injured is up to 515, according to the Associated Press.

A country music event outside Las Vegas' Mandalay Bay Resort and Casino was hit by by a gunman's deadly attack on Sunday night (October 1), leaving more than 50 dead and 400 injured, Las Vegas police reported (per the New York Times.) It's now the deadliest mass shooting in U.S. history.

During a set by country star Jason Aldean at the outdoor Route 91 Harvest Festival, the gunman, who police identified as Stephen Paddock, 64, opened fire upon the crowd from a hotel before reportedly killing himself. Footage from the scene shows Aldean fleeing the stage amid the sound of prolonged, rapid gunfire. Las Vegas police said 22,000 people were in attendance; the large crowd ducked for cover, then fled in a scramble at the sound of the shots.

One witness, Robyn Webb, described it to the Las Vegas Review-Journal as "relentless." "It just kept coming," Webb said, the Times reports.

Aldean called the night "beyond horrific" in an Instagram post from after the attack. "I still don't know what to say but wanted to let everyone know that me and my crew are safe," he wrote. "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."

This marks the third major attack at a music event in two years. In November 2015, terrorists opened fire at the Bataclan concert hall in Paris during an Eagles of Death Metal concert in a coordinated assault that also targeted cafes, restaurants, and a sports stadium. In total, 130 people were killed, including 89 at the Bataclan.

And just five months ago in May 2017, a terrorist explosion at an Ariana Grande concert at England's Manchester Arena killed 22 people and injured 250.

The Las Vegas attack has also overtaken June 2016's deadly Pulse nightclub shooting in Orlando as the worst mass shooting in American history.

President Trump tweeted out his "warmest condolences" to the victims of the attack on their families on Monday morning.

This story is developing.