Authorities planned to use videos, witness interviews and a review of concert procedures to figure out what went wrong on Friday night during a performance by rapper Travis Scott.

The tragedy unfolded when the crowd rushed the stage, squeezing people so tightly they couldn't breathe.

Billy Nasser, 24, said about 15 minutes into Scott's set, things got "really crazy" and people began crushing one another. He said he "was picking people up and trying to drag them out".

Nasser said he found a concertgoer on the ground.

"I picked him up. People were stepping on him. People were like stomping, and I picked his head up and I looked at his eyes, and his eyes were just white, rolled back to the back of his head," he said.

Over the weekend, a makeshift memorial of flowers, candles, condolence notes and T-shirts, including a Scott shirt, took shape outside the outdoor venue where the concert was held.

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. The youngest was 14, the oldest 27.

Houston officials did not immediately release the victims' names or the cause of death.

Thirteen people remained hospitalised on Sunday. Their conditions were not disclosed. Over 300 people were treated at a field hospital at the concert.

City officials said they were in the early stages of investigating what caused the pandemonium at the sold-out Astroworld festival, an event founded by Scott. About 50,000 people were there.

Authorities said that among other things, they will look at how the area around the stage was designed.

Julio Patino, of Naperville, Illinois, who was in London on business when he got a middle-of-the-night call informing him his 21-year-old son Franco was dead, said he had a lot of questions about what happened.

"These concerts should be controlled," Patino said. "If they don't know how to do that, they should have cancelled the concert right then, when they noticed there was an overcrowd.

"They should not wait until they see people laying down on the floor, lifeless."

Australian Associated Press