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A crush of fans pushing toward the stage during rap star Travis Scott's Astroworld music festival in Houston has killed at least eight people and injured dozens more.
City Police Chief Troy Finner said his department has opened a criminal investigation by homicide and narcotics detectives following unconfirmed reports someone in the audience "was injecting other people with drugs".
The disaster unfolded around 9.30pm local time on Friday during the headline performance by Scott, a Grammy-nominated singer and producer, following what police and members of the crowd described as an escalation of unruly behaviour.
As some in the sold-out audience of about 50,000 surged toward the stage, people began to fall unconscious, some apparently suffering cardiac arrest or other medical issues, officials told reporters outside the venue.
Minutes later the chaos was declared a "mass casualty incident".
"It happened all at once. It seemed like it just happened ... over the course of just a few minutes," said Houston Police's Larry Satterwhite, who was at the front of the event when the situation began.
Satterwhite said he immediately met with promoters and they agreed to halt the show. Officials said the concert was brought to an end by 10.10pm.
Finner said among the "narratives" under review by police were reports suggesting "some individual was injecting other people with drugs".
One report involved a security officer "who felt a prick in his neck" as he was trying to restrain or grab someone and then fell unconscious, only to be revived with a dose of the opioid antidote NARCAN, Finner said, citing an account from medical personnel who treated the officer.
Finner said the medical staff also noticed what appeared to be a needle mark on the officer's neck.
It was not clear whether authorities suspected such an episode played a role in the crowd surge, but Finner said, "we're going to get down to the bottom of it".
City Fire Chief Samuel Pena said it appeared the venue had ample exit routes for fans and none were obstructed.
Twenty-five people were taken to hospitals by ambulance after the crush began, some of them in cardiac arrest, with 13 still hospitalised on Saturday - five under age 18.
Eight people died. Most ranged in age from 14 to 27, though one victim's age was not immediately known.
After mobbing entrance gates and merchandise booths early in the day, the crowd grew increasingly unruly as performers took the stage, according to 19-year-old festival goer Hamad Al Barrak.
"There were just too many people,' said Al Barrak, who described chaos as he tried to buy festival gear. "We were all pressed together. You felt like you couldn't breathe."
Albert Merza, 43, part of a group of eight people from Detroit attending the festival, recounted seeing "a lot of drinking and crazy behaviour".
"It felt like a riot," he said, adding that it appeared about half the crowd were under age 21. "There were people throwing out stuff, objects flying everywhere."