HOUSTON (AP) — The crowd at a Houston music festival suddenly surged toward the stage during a performance by rapper Travis Scott, squeezing fans so tightly together that they could not breathe or move their arms, witnesses said Saturday, hours after at least eight people died in the chaos.

The pandemonium unfolded Friday evening at Astroworld, a sold-out, two-day event in NRG Park. An estimated 50,000 people were in attendance.

Niaara Goods, 28, said the crowd surged as a timer clicked down to the start of the performance.

"As soon as he jumped out on the stage, it was like an energy took over and everything went haywire. All of a sudden, your ribs are being crushed. You have someone's arm in your neck. You're trying to breathe, but you can't," Goods said.

Goods said she was so desperate to get out that she bit a man on the shoulder to get him to move.

The dead ranged in age from 14 to 27, and 13 people were still hospitalized, Mayor Sylvester Turner said. He called the disaster "a tragedy on many different levels" and said it was too early to draw conclusions about what went wrong.

Houston Fire Chief Samuel Peña said the crowd's movement toward the stage caused panic and some injuries. Then "people began to fall out, become unconscious, and it created additional panic."

Experts who have studied deaths caused by crowd surges say they are often a result of density — too many people packed into a small space. The crowd is often running either away from a perceived threat or toward something they want, such as a performer, before hitting a barrier.

G. Keith Still, a visiting professor of crowd science at the United Kingdom's University of Suffolk, has testified as an expert witness in court cases involving crowds. He said he usually does not look at eyewitness reports when in the early stages of analyzing an incident because emotions can cloud the picture, and witnesses can see only what's immediately around them.

The deaths called to mind a 1979 concert by The Who where 11 people died  as thousands of fans tried to get into Cincinnati's Riverfront Coliseum. Other past crowd catastrophes include the deaths of 97 people in an overcrowded Hillsborough Stadium in 1989 in Sheffield, England, and numerous disasters connected with the annual hajj in Saudi Arabia.

People in the Houston crowd reported lots of pushing and shoving during the performances leading up to Scott's set.

Then when Scott took the stage, the crowd seemed to rush to the front, trying to get closer to the stage, said Nick Johnson, a high school senior from the Houston suburb of Friendswood who was at the concert with friends.

"Everyone was passing out around you, and everyone was trying to help each other. But you just couldn't move. You couldn't do anything. You can't even pick your arms up," Johnson said. "It just got worse and worse."

Johnson said fans started to crush each other, and people started screaming. He said it felt like 100 degrees in the crowd.

Scott seemed to be aware that something was going on in the crowd, but he might not have understood the severity of the situation, Johnson said.

On video posted to social media, Scott could be seen stopping the concert at one point and asking for aid for someone in the audience: "Security, somebody help real quick."

In a tweet posted Saturday, Scott said he was "absolutely devastated by what took place last night." He pledged to work "together with the Houston community to heal and support the families in need."

Houston Police Executive Assistant Chief Larry Satterwhite, who was near the front of the crowd, said the surge "happened all at once."

"Suddenly we had several people down on the ground, experiencing some type of cardiac arrest or some type of medical episode," Satterwhite said. "And so we immediately started doing CPR and moving people right then."

Satterwhite said promoters quickly agreed to end the event "in the interest of public safety."

Amy Harris, a freelance photographer for The Associated Press, described an "aggressive" crowd atmosphere throughout the day because of the way fans were behaving — pushing and rushing the stage barricades and prohibited VIP and admission areas.

"It was definitely the most chaotic festival environment that I've been in," Harris said. "I felt uneasy all day."

She got trapped behind a barricade while photographing performer Don Toliver because about 300 fans rushed the area. They ended up behind the security barricade with her.

She encountered a similar scene at a different stage for the main act. She left the media pit after three songs because of the disorder, which resulted in people being pulled over the security barricade to receive medical attention.

At one point, Gerardo Abad-Garcia was pressed so tightly into the crowd that he could not move his arms off his chest. During Toliver's performance, which came before Scott's appearance, he started getting concerned for his safety.

"I just couldn't breathe. I was being compressed," he said. A security guard helped him and others climb a fence and get out.

He described the crowd during Scott's set as a wave that was "going forward and backward."

Some people lost their shoes, and the ground was littered with clothing and trash. He said some people tried to help those who were passed out on the ground, while other concertgoers seemed to ignore them and continued watching the show.

After Scott's concert, Abad-Garcia saw medical personnel performing CPR on someone who appeared to be unconscious as the person was taken away on a golf cart.

Authorities did not disclose the causes of death, and the dead were not immediately identified.

Scott, one of music's biggest young stars, founded the Astroworld Festival in 2018. The 29-year-old Houston native has been nominated for eight Grammy Awards. He has a 3-year-old daughter with Kylie Jenner, who announced in September that she's pregnant with their second child.

Drake joined Scott on-stage at the concert, which was livestreamed by Apple Music.

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Associated Press writers Ryan Pearson in Los Angeles, Stan Choe in New York, David Sharp in Portland, Maine, and Desiree Seals in Atlanta contributed to this report.