For Jaelyn Duran, the Route 91 Harvest Festival was a last-minute excursion. The three-day country music festival had been sold out for weeks, but she; her sister, Sarah Duran; and her sister's boyfriend, Billy, managed to secure 11th-hour tickets to the event, which attracted more than 22,000 fans to the famous Las Vegas Strip.
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"It was a really, really good day," Jaelyn Duran, 18, told ABC News, sharing how she took off from her job as a restaurant server to attend the concert Sunday. It was the first time that she and her sister attended a concert together, and they listened to a few acts throughout the evening and danced at a silent disco before pushing through the crowd to get as close to the stage as possible for a performance by the event's headliner and one of the sisters' favorite artists, Jason Aldean.
But the fun evening turned deadly when suspect Stephen Paddock, 64, opened fire at concertgoers from the 32nd floor of the Mandalay Bay Resort and Casino across the street.
At first, Jaelyn Duran said she had no idea what was happening; she had never heard gunfire before. But when Aldean turned away from his microphone and abandoned the stage, confusion turned to fear.
"When he ran off, everyone kind of panicked," Duran said. It was at that moment, as bullets pierced the air in a rapid staccato, that she realized she was in the middle of a shooting — one that would soon become the deadliest in modern U.S. history, claiming 59 lives and injuring more than 500 people.
As the venue's lights flashed on, illuminating the festival's open-air site, Duran looked next to her to see a woman she recognized. Moments earlier they had been standing beside each other, singing and laughing as Aldean performed. Now the woman was on the ground, bleeding out from a wound to her back.
"I'm only 18. I shouldn't be seeing stuff like that," said Duran.
Suddenly, clutched by her sister and Billy, Jaelyn Duran was running away from the stage. They worked their way through the crowd and hopped a fence, with her cowboy-booted feet landing in a puddle of blood. At that moment, the gravity of the situation began to sink in, she said.
"I was hysterical. I was crying," she said, recalling the sound of bullets ricocheting off the fence behind her.
The group took off again, trekking farther through the masses and scaling a second fence. Duran's knees were cut, and her legs were covered in dirt and blood as she ran past a concession stand, where the three were captured fleeing by a Getty Images photographer.
They eventually came upon a garbage receptacle and hid inside. Duran said she quickly called her father but could barely talk to let him know she was OK. From there, they spotted an open gate to the adjacent McCarran International Airport and made their way to a nearby hangar, where other concertgoers had congregated; at least two had bullet wounds, she said.
Those at the hangar were eventually transported to the Thomas and Mack Center at the University of Nevada at Las Vegas, where Jaelyn Duran, Sarah Duran and Billy were picked up by his mother.
"I finally felt safe," Jaelyn Duran said.
Twenty hours later, she said she still hasn't been able to fall asleep and knows the memories of Sunday will stick with her the rest of her life. She repeated a sentiment voiced all too often by those affected by mass shootings.
"You always think this will never happen to you," she said.
ABC News' Mya Green contributed to this report.