HOUSTON, United States (AP) — Authorities said they would watch video, interview witnesses and review concert protocols to determine how eight people died at Astroworld.

The Houston music festival on Friday night saw fans suddenly surging toward the stage to watch rapper Travis Scott. City officials said Saturday they were in the early stages of investigating the pandemonium that unfolded at the sold-out event in NRG Park with an estimated 50,000 people in attendance.

The dead ranged in age from 14 to 27, and 13 people were still hospitalized Saturday, 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.

 “It may well be that this tragedy is the result of unpredictable events, of circumstances coming together that couldn’t possibly have been avoided,” said Judge Lina Hidalgo, Harris County’s top elected official. “But until we determine that, I will ask the tough questions.”

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.

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.

Based on fire codes, the venue could have held 200,000 people, but city officials limited the attendance to 50,000, Houston Fire Chief Samuel Peña said.

“It was the crowd control at the point of the stage that was the issue, especially as the crowd began to surge toward the stage,” Peña said.

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.