More than 50 people were killed and 400 injured when a lone gunman opened fire from a perch high up in the Mandalay Bay Resort and Casino in Las Vegas Sunday night, police said, making it the deadliest shooting in modern U.S. history.

The "nonstop gunfire," according to one witness, sent bystanders outside the resort on the Vegas strip ducking for cover and scrambling for their lives.

Bystanders sprang into action, caring for the wounded and at least one described someone dying in their arms.

Police said a Las Vegas police officer who was off-duty attending the concert is among the dead.

One video showed the terrifying aftermath as the injured lay on stretchers or on the ground with responders and bystanders surrounding them to give aid. Bystanders made makeshift stretchers out of police barricades, plugged wounds with their hands and used their clothing to try to stanch the bleeding from the wounded.

Videos filmed by onlookers gave a window into the chaos that ensued, with some thinking that fireworks were going off. The final night of the Route 91 Harvest Music Festival was taking place across the street from Mandalay Bay. Jason Aldean was the night's headliner; other performers included country stars Jake Owen, Big & Rich, Luke Combs and Dylan Scott.

In the wake of the shooting, the Las Vegas Police Department said the suspected gunman, who was believed to be a local resident, was on 32nd floor of Mandalay Bay hotel. Police responded to the scene, engaged him and he is now dead. Law enforcement officials identified the suspect as 64-year-old Stephen Paddock. The motive was unclear.