Thousands of phones are held aloft, their lights flickering as crowds sing "God Bless America" along with duo Big & Rich.
More than 22,000 people are here at the Route 91 Harvest Festival but it's like a family gathering.
Bartender Heather Gooze knows customers by name, laughing and joking in the party atmosphere all around.
Performer Bryan Hopkins takes photos with fans before squeezing into the front rows to watch the final act, Jason Aldean.
Caren Mansholt and her boyfriend grab drinks and settle into bleachers to the right of the towering open-air stage.
Michael and Jamie Goguen are at the festival for the third time. They're in the VIP section and basking in their vantage point.
Bowdien Derby apologizes after spilling beer on a young woman. His brother-in-law Madison Viray laughs with him before heading further from the stage for a quick catnap on the AstroTurf. He wakes to the roar as Aldean takes the stage.
It is a little after 10 on Sunday night.
Across the street and 32 floors up, a man is preparing to become a mass murderer.
Caren Mansholt is on her feet, clapping along, when a rapid series of pops bursts through the air.
Fireworks, she thinks, and looks to the sky to see them. Nothing is there. Strange. Back to the music. Aldean is playing "When She Says Baby."
Then more shots come. More and more. Aldean and his band rush off the stage and the crowd realizes something is terribly wrong.
The gunfire is loud. Some people start running for exits. Others are already hurt.
"There was just a spray of [gunfire] and two guys go down right in front of us," Hopkins, the musician, says. "I look back and the girls that are singing behind us go down."
Mansholt's boyfriend, Rusty Dees, screams at her to get down. They can tell the gunfire is to their right. But they think the gunman is inside the festival. Dees kicks off his flip-flops and runs toward where he thinks the gunshots are coming from to try to help, or to stop them.
Mansholt is lying between the bleacher seats, screaming his name, too terrified to lift her head. She doesn't want to be in the line of fire.
Closer to the stage, everyone is on the ground. But Derby fears they could be sitting ducks like that. He, too, believes the shooter or shooters are on the festival grounds, perhaps heading their way.
He sees his cousin lying on top of his girlfriend to shield her. He turns to his aunt. "I'll never forget the look in her eyes," Derby says. "It was the look of, are we about to die? Is this it?"
The firing doesn't stop.
On the 32nd floor of the Mandalay Bay hotel, Stephen Paddock is the reason.
The 64-year-old has turned the luxury suite into a sniper's nest. Twenty-three weapons, some with scopes, are in the room with him, magazines of ammunition stacked neatly on the floor.
Long guns are on bipods. They allow him to aim more easily and to fire at will onto the crowd. Many of the rifles have bump stocks. He can now effectively shoot them as automatic weapons.
Cameras in the hallway are positioned so he can see who is coming. One is on a service cart. Another through the suite's peephole.
He's used a hammer to smash two windows, giving him a clear view of the festival and its crowds below.
He sprays off a round of bullets. Then another series of rapid fire shots. His weapons are not designed for this intense fire and will have become incredibly hot. He does not stop.
On the ground, the music fans of five minutes ago are now prey. They wait for a lull in the gunfire so they can bolt. They run and hide. They duck. They dart. And then gunfire erupts again. They scream.
A bullet whizzes through a man's baseball cap, near its Quiksilver logo. It just misses his head.
Many people are down on the ground.
Hopkins runs to what he thinks is an artists' entrance by the stage but he can't find a way out. Instead he sees a freezer, maybe four or five feet high and rushes people inside, repeating to all, "It's going to be OK." To a friend with him, he swears, "This is not where I'm going to die."
Now Mansholt and her boyfriend are trying to make it as far as they can from the venue. But the constant barrage of gunfire stops them again and again. They run. They hear rapid fire. They hide. Once behind a tree. Again after passing a gas station as they neared the Tropicana resort parking lot. They come across a man with his grandson, maybe 3 years old, on his shoulders. They tell them to take cover behind a concrete wall.
She's in cowboy boots, he is barefoot.
Derby is also running with his family. "A few seconds in to us trying to get away, you could hear him start rattling off again," he says.
He sees a tent, a covered bar and grabs the hands of some young women to get them inside. He looks for his aunt, his cousin, his girlfriend -- but they're not there. They've lost each other.
The people with Derby are frozen. Crying. And afraid they could be targeted next. They run again, toward Las Vegas Boulevard, the Strip.
Heather Gooze has left that same bar where she was working, called to help move an injured man to a sidewalk. She kneels and holds his hand.
"I felt, like, a squeeze on my fingers, and then I just felt the fingers go loose," she says. Jordan McIldoon dies on that sidewalk but Gooze doesn't leave. She doesn't want him to be alone.
So many people are on the ground now. Off-duty firefighters are performing CPR and starting to triage victims inside the festival.
As people get out, scaling walls or pushing down fences, police officers try to direct them to safety.
"Go that way! Go that way! Stay down!"
A few minutes have passed and the city's first responders are getting alerts. Surgeons and nurses pick up ringing phones. Get to the hospital as soon as you can.
Still, the bullets are raining down.
Michael and Jamie Goguen have made it to a cinderblock wall. Michael, a helicopter rescue pilot, won't leave. Jamie won't leave him. They start to help.
They carry victim after victim to the festival's medical tent that's now a triage center. One man with a head wound doesn't make it that far, dying in their arms on the field.
They flag down cars and trucks.
Jamie tells them: "I need to put shot up people that are bleeding and hemorrhaging into your car and you're going to take them to the hospital right now."
Again and again, they cram people in. One police officer is holding a compression bandage tightly to a wound on his own neck with his left hand. He uses his right hand to help Michael shuttle another victim to a car.
Inside the Mandalay Bay, a police officer radios to base: "I'm inside the Mandalay Bay on the 31st floor, I can hear automatic fire coming from one floor ahead ... one floor above us."
On the 32nd floor, Brad Baker is woken by loud noises. As he tosses and turns, he figures it's the concert below.
Then two police officers with guns and flashlights barge into this room and tell him to get out.
"One officer with his AR going down the hallway and another with a shotgun right next to my door," he says. "It looked like providing cover. And they told me to get out and hug the wall and run as fast as I could to the elevator bay."
There, five or six people are already huddled together, comforting a 4-year-old boy.
"I saw the little boy and thought about my own daughter and told myself don't risk it, just get out as fast as you can, when you can," Baker says.
Down the hallway, Jesus Campos, a hotel security guard, gets too close to the door to Room 32135. Paddock shoots him through the door.
Dr. John Fildes gets to the University Medical Center -- the only Level One trauma facility for Las Vegas -- about 15 minutes after getting the call.
Nurses, surgeons and specialists in their scrubs are triaging the first wave of wounded in the ambulance bay.
Patients on gurneys fill the hallway. There is blood on all the floors.
The medics have clear goals -- prioritize the most critical cases to get them into surgery and make space for more patients, who just keep coming and coming -- more than 100 to this hospital alone.
"We got patients with clear indications to go to surgery, they wasted no time going into the resuscitation area," says Fildes.
There are bullet wounds to chest, torso and limbs. Bones are shattered, tissue shredded.
There are injuries from when people fell, or were trampled. Others were hit by cars as they ran into the road to escape.
Musician Hopkins decides to bolt. He leads his group from the freezer and finds a makeshift ramp across a fence where people are hopping over.
A police officer sweating and screaming takes off toward the shooting and shouts for them to go the other way, away from the festival area.
"We start running ... and there's a body, a body, a body and then there's another body and this time the guy is shot in the stomach and his friends are there pumping on his chest trying to resuscitate him and one of the girls starts to panic," he says.
She wants to call her father.
At last the shooting stops. Roughly 10 minutes that seemed like an eternity.
The police change tactics -- as long as there isn't active firing they can take a little time. They know which room the shooter is in, but not what is there. They gather a team to force their way in to what could be an explosive booby trap.
"Breach! Breach! Breach!" and the thick hotel door is forced open. It's a little over an hour after the gunfire stopped.
Inside, a corpse of a man lies on the gray striped carpet, apparently killed by his own hand. He is surrounded by his arsenal, bullet casings everywhere.
On the neon-lit streets below, the danger does not feel like it's over.
"Did you hear that there's car bombs?"
"Shooter!" someone shouts inside the Hooters Casino Hotel where people are seeking refuge.
That starts a stampede, and Hopkins -- still with the couple of dozen people he sheltered with in that freezer -- leads them again, this time to the kitchen where they lock themselves in.
Nowhere feels safe. People are lost, separated from their families and minutes turn into hours.
Gooze is told it's time to leave the body she's been sitting with for hours.
But there are reunions, too.
Brothers-in-law Bowdien Derby and Madison Viray finally connect on the phone and meet up at the Tropicana, then move to the Hooters hotel. There's an area for wounded victims and firefighter Viray goes to help.
After 25 minutes, Hopkins comes out of the kitchen to a scene that is surreal in its normalcy. "People are gambling and sitting around drinking," Hopkins says.
He's confused and exhausted. He makes it home about 4 a.m. and plugs in his phone, which had died, and sees messages from friends. He posts on Facebook that he's OK.
Caren Mansholt and Rusty Dees make it to the Cosmopolitan, where they are staying. They watch some news of the massacre, but it's too much.
They are among the thousands who saw such hell. They are also among the thousands that saw and performed such heroics to help friends and strangers alike.
They think they should try to sleep, but they're worried the nightmare isn't over. They go to bed wearing tennis shoes.
"We were fully dressed ready to escape again."
CNN's Ray Sanchez contributed to this report.