Las Vegas Shooting: Death Toll Rises to 20 in Mandalay Bay Attack

The "nonstop gunfire," according to one witness, sent bystanders outside the resort on the Vegas strip ducking for cover and fleeing for their lives. Tourists hid in their hotel rooms and flights headed into the McCarran International Airport were held elsewhere, ABC News reported.

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

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 when the shooting took place and concertgoers were caught off-guard.

In the wake of the shooting, the Las Vegas Police Department said the suspected gunman, was believed to be a local, was on 32nd floor of Mandalay Bay hotel. Police responded to the scene, engaged him and he is now dead. Officials said they did not believe there were any more shooters.

Authorities were looking for a companion of the shooter, Marilou Danley, as well as two vehicles, a Hyundai Tucson and Chrysler Pacifica, both with Nevada plates. At least two police officers were hospitalized.

"We were just at the concert there, and Jason Aldean was playing," one of the concertgoers, named Mike Cronk, 48, a retired teacher, said. "Kinda sounded like some fireworks going off. I think there was the first kinda volley, and then all of the sudden second volley. My buddy's like 'I just got hit, ya know.' He got hit three times. Then people started diving for the ground. And it just continued."

"It was pretty much chaotic," Cronk continued. "Lots of people got hit. ...It took a while to get him out. We had to get him over the fence and hiding under the stage for a while, ya know, to be safe. And finally, we had to move him because he had three chest wounds."

Cronk said his group were finally able to track down an ambulance "and basically the one guy ended up dying in my arms because he was bleeding," he said. "And my buddy got in there. We got three more people in the ambulance. ... But I just got a message from my buddy -- and he's going to be okay."

As bursts of gunfire crackled in the air, people outside of the casino ducked and screamed, according to video filmed by witnesses.

"We're going to get trampled if we don't go," a bystander could be heard saying in a dramatic video of the incident. Confusion appeared to abound as those outside fled the scene with another person saying, "it's fireworks."

Michelle Leonard, who was located in a booth near the main entrance of the arena, said the shooting just “kept going nonstop.”

Leonard said “mass confusion” unfolded as people tried to flee the scene. She said the shooting seemed like it went on for more than a minute.

“I had no idea of where it was coming from or where to run to," Leonard said.

She said she injured her leg as she tried to escape.

Another witness, Jake Freeman, said he was standing on the rooftop of a nearby hotel when the shooting broke out.

“I had a bird’s-eye view” of the shooting, Freeman said. He said he saw "crowds of people running” as people “dropped to the ground.”

“At the moment we didn’t realize that they were being shot,” he said.

Aldean, who performed at the country concert Sunday night, posted an image of Las Vegas on Instagram and wrote "Pray for Las Vegas."

ATF (Bureau of Alcohol Tobacco and Firearms) agents have responded to the scene and the FBI is assisting with the investigation.

The Las Vegas Police Department said people should avoid the area. Flights in and out of the McCarran International Airport in Las Vegas were temporarily halted due to the incident.

The FBI is also supporting local law enforcement efforts in Las Vegas.

"Pray for Las Vegas. Thank you to all our first responders out there now," Las Vegas Mayor Carolyn G. Goodman said via Twitter.