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HOUSTON (NewsNation Now) — A 56-page event operations plan for the Astroworld music festival included protocols for dangerous scenarios including an active shooter, bomb or terrorist threats and severe weather, but it did not include information on what to do in the event of a crowd surge.

But that’s what authorities believe happened Friday night when eight people died after headliner Travis Scott took the stage at the outdoor festival in Houston that is now the focus of a criminal investigation. Authorities have said 50,000 people attended the event.

“In any situation where large groups of people are gathering, there is the potential for a civil disturbance/riot that can present a grave risk to the safety and security of employees and guests,” the plan said. “The key in properly dealing with this type of scenario is proper management of the crowd from the minute the doors open. Crowd management techniques will be employed to identify potentially dangerous crowd behavior in its early stages in an effort to prevent a civil disturbance/riot.”

If crowds are displaying threatening or destructive behavior, security and a supervisor should be notified, the plan said.

Experts say crowd surge deaths happen because people are packed into a space so tightly that they are being squeezed and can’t get oxygen. It’s not usually because they’re being trampled.

None of the people listed in charge of managing Astroworld’s security and operations have responded to requests for comment.

This comes as the FBI has joined a criminal investigation into what caused the pandemonium at the sold-out Astroworld festival. City officials Tuesday were in the early stages of investigating what caused the chaos as a prominent local official called for a separate, independent review of the tragedy.

Both the city’s police and fire departments were deeply involved in the safety measures for the festival, playing key roles in crowd control measures, on-site security staffing and the emergency response. The police chief even says he met with headliner Travis Scott before the show.

One of the youngest injured in the chaos was 9-year-old, Ezra Blount. He is in an induced coma with brain injuries after he fell from his father’s shoulders as the crowd surged during the festival. Ezra was trampled by the crowd.

Lola Lingos and her friend Grace Williams were two of the estimated 50,000 people in attendance at Astroworld, Videos taken by friends show Lingos being crushed by the crowd.

“We couldn’t give over the fence. I had to have a guy pick me up and throw me over the fence onto the ground,” said Lingos.”I think I didn’t even land on my feet. I had to have somebody literally pick my whole body up and throw me to get out.”

The pair said they saw three lifeless bodies and no medics in the area to help.

“Everybody was screaming stop the show, stop,” Lingos said. “Nobody was doing anything, like everybody was trying to stop it but nothing was happening.”

Williams said that teenagers were the ones performing CPR because there were no medics to help.

Houston Police Chief Troy Finner has defended how long it took for the concert to be called off after the first signs of trouble. The police chief said his department immediately notified concert organizers after noticing that attendees were “going down.” The event was called off 40 minutes later after discussions that included the fire department and NRG Park officials.

“You cannot just close when you’ve got 50,000 — over 50,000 — individuals, OK?” Finner said. “We have to worry about rioting — riots — when you have a group that’s that young.”

Houston Police Department spokeswoman Jodi Silva declined to comment on questions about whether its close involvement in the event created a conflict or if it considered handing the probe off to an outside agency. Such decisions are often made in investigations like police shootings.

The police department’s probe would be separate from any independent investigation ordered by County Judge Lina Hidalgo, Harris County’s top elected official, according to Rafael Lemaitre, a spokesman for the county judge’s office. Hidalgo hasn’t decided who would conduct such an independent review or how it would be done, Lemaitre said Monday.

The dead, according to friends and family members, included a 14-year-old high school student, a 16-year-old girl who loved dancing and a 21-year-old engineering student at the University of Dayton. The youngest was 14, the oldest 27.

On Monday, Variety reported that Scott will be issuing refunds to all attendees of the festival and he wouldn’t be performing at an upcoming Las Vegas festival this weekend.