LAS VEGAS • At first, it sounded like fireworks - a loud, crackling noise. Then the awful realisation began to spread, unevenly, through the huge crowd. It dawned on people when they heard screams, when they saw bloodied victims collapse around them, or when others stampeded for the exits, trampling those in their way.
More than 22,000 people had been at the Route 91 Harvest festival, a three-day country music concert with grounds across the street from the Mandalay Bay resort, when the shooting began about 10 pm on Sunday, according to police.
As country star Jason Aldean played what was expected to be one of the last sets of the night, a gunman opened fire - his bullets flying from a window on the casino's golden facade, which he had smashed with some type of hammer.
Many of the terrified concertgoers followed their instincts and crouched or lay flat, not realising that they remained exposed to a gunman lodged high above them.
"People were getting shot at while we were running, and people were on the ground bleeding, crying and screaming. We just had to keep going," said 28-year-old Dinora Merino, a dealer at the Ellis Island casino who was at the concert with a friend. "There are tents out there and there's no place to hide. It's just an open field."
Disbelief quickly gave way to reality as volley after volley of rapid gunfire rained down. The terrifying moments were recorded by many social media users. "I'm pretty sure a terrorist attack just happened next to me. I'm at Mandalay Bay," Las Vegas Twitter user @GLOKMIN tweeted early on in the assault.
Out on the streets, taxi drivers received word of the danger within minutes when an advisory of police activity came across the blue screens on their taxi meters.
Citizens turned their private vehicles into ambulances, and others became on-the-spot medics, using metal crowd-control stanchions as stretchers.
"We have a head shot. We have a head shot," one man yelled in a video of volunteer medics at work.
Ms Lindsay Lee, a self-described Las Vegas tattoo model, turned her truck into an ambulance when a man at the scene asked her to take victims to the hospital.
"Go ahead, put them all in the back," she said in one video. Then came a moment that few volunteers could be prepared for.
"One died in the back of the truck and my fiance had to carry him inside the hospital," Ms Lee said on Instagram. "This was a nightmare. I've never ran for my life like I did tonight."