At first, they say, it seemed like just part of the music.
It was a Saturday night and the dance tracks had been pounding at the Pulse Orlando nightclub, which calls itself the city's hottest gay bar. A sharp, staccato sound, arriving shortly after 2 a.m. ET, didn't seem out of place.
"I thought it was a Ying Yang Twins song or something," club-goer Christopher Hansen told reporters. "It went with the beat almost."
Luis Burbano, who was also inside the Orlando nightclub, told ABC News that at first he thought the noise was the DJ's doing — just a sound effect.
Rosie Feba's girlfriend thought the sounds were shots. But even as people around them were getting down on the floor, Feba wasn't convinced, she told the Orlando Sentinel.
"I told her I didn't think it was real — I thought it was just part of the music," she said.
Ray Rivera, one of the DJs that night, knew it wasn't his music. He was playing reggae on the patio, he told The New York Times, when he heard the sounds. For his part, he thought at first it was firecrackers.
He turned the music down a bit and listened closer. Then he realized, and he ran.
"You heard just too many shots," Hansen said. "That's when you just know ... just one after another after another." He dropped to the ground, crawling on his elbows and knees.
For Feba, it didn't sink in until she saw the flash from the shooter's gun.
An armed man, whom law enforcement officials have since identified as Omar Mateen, was firing on the crowd in the packed club.
In the confusion of the dark, packed club, people dropped to the ground and hid in the bathrooms. Many started to run for the exits, grabbing friends and strangers along the way.
"I don't remember screaming. I don't even remember breathing," Burbano told ABC News. "I just remember dragging my best friend down."
It was hard to leave the club, he told the network, because the floor was covered with drinks and blood.
The shots kept coming.
Brandon Wolf was in the bathroom of the club, hearing the shots get closer and closer, he told the Orlando Sentinel. He and his friends finally decided they had to run for it.
"All I heard was gunfire after gunfire," Wolf said. "Eventually, I thought you were supposed to run out of ammunition. But it just kept going and going."
"It was just one after another after another after another," Hansen told reporters. "After everyone was out, the shooting was still going, and the cops were yelling, 'Go, clear the area!' "
Survivors gathered across the street, shocked and confused, and did what they could to help the wounded.
Feba grabbed a man who had been shot, as other club-goers called 911, she told the Sentinel.
Burbano told CNN he saw another survivor of the attack collapse outside the club, and grabbed him only to discover that the man's forearm was "split in two." Burbano said he ripped off his shirt and tied it around the injury.
As first-responders worked to treat the dozens of victims, the crisis continued inside Pulse. There was a shootout between police and the gunman — but then the shooter returned inside the building, where trapped club-goers effectively became hostages.
Mina Justice's 30-year-old son, Eddie, was among them. He texted her, asking her to call police, The Associated Press reports:
"He told her he ran into a bathroom with other club patrons to hide. He then texted her: 'He's coming.'
" 'The next text said: "He has us, and he's in here with us," ' Mina Justice said. 'That was the last conversation.' "
The hostage situation stretched on for hours, with some hostages reportedly contacting 911 from inside the building. Around 5 a.m., police used an armored vehicle and a controlled explosion to break into the building and engage the shooter in another firefight.
This one ended in the shooter's death, the rescue of 30 hostages — and the horrifying discovery that the death toll was even higher than police had feared.
After previously estimating that about 20 people had died in the attack, authorities found far more bodies inside the club. The death toll was raised to 50, making the attack the deadliest mass shooting in modern American history.
The horrifying arc of the night — from party scene to carnage — was documented on Facebook by Anthony Torres.
Shortly after midnight, he uploaded a video of two men dancing exuberantly to pounding music, beneath bright lights, as other club-goers passed by with drinks.
"Omg it's been so long since I have dance with Alfredo Chavarria," he wrote, followed by four hearts.
Just over two and a half hours later, he posted new videos — of sirens, running cops, the sound of gunshots.
He was still there after 5 a.m., when the police stormed the building. And he recorded that, too.
"They're shooting back and forth," a voice says on Torres' video.
"Look at this. Oh my God."
Editor's note: We've updated the text to reflect that the Orlando attack represents the deadliest mass shooting in modern U.S. history, rather than in all of U.S. history. You can read more about our thinking here.