At least eight people died and scores of people were hurt after a crowd surge on the opening night of the music event in Houston, Texas.
The victims were aged between 14 and 27. The identities of some of them are expected to be released on Sunday
Police are also investigating reports that somebody in the audience had been injecting people with drugs.
The incident began around 21:15 on Friday (02:15 GMT Saturday) when panic broke out as the crowd began to press towards the front of the stage during the rapper Travis Scott’s headline set.
As the crush began causing injuries to people, panic grew and the casualties quickly overwhelmed the on-site first aiders, officials said. Some 300 people were treated for injuries such as cuts and bruises.
The police investigation into the tragedy will involve the homicide and narcotics divisions, and will review video from the scene to explore the causes of the surge and what had prevented people from being able to escape.
Several concert goers had to be revived with the anti-drug overdose medicine, including a security officer who police said appeared to have an injection mark.
“We do have a report of a security officer… that he was reaching over to restrain or grab a citizen and he felt a prick in his neck,” Houston Police Chief Troy Finner said.
“When he was examined he went unconscious,” he added. “He was revived and the medical staff did notice a prick that was similar to a prick that you would get if somebody is trying to inject.”
In his first statement since the event on Twitter, Travis Scott thanked the police and emergency services and said he was “committed to working together with the Houston community to heal and support the families in need”.
Later he posted a video message on Instagram, in which he encouraged anyone with information about the incident to contact the authorities.
Scott, whose real name is Jacques Webster, made his big breakthrough in 2013 and has since had eight nominations for Grammy Awards. He has a child with celebrity socialite Kylie Jenner, who was among the 50,000 people attending the concert.
In 2018, he pleaded guilty to a public disorder charge after he was accused of encouraging people to rush the stage at a concert in the US state of Arkansas.
According to a local newspaper, Travis Scott also paid nearly $7,000 (£5,186) to two people who said they were injured at the event.
Sense of belonging turned deadly
The night wasn’t supposed to end like it did. Today, the streets surrounding the Astroworld festival grounds are still mostly closed off, and the venue is almost empty except for staff and a heavy police presence.
He was right in the middle of the crowd when, he says, things began to get ugly. As the surge grew worse in a matter of minutes, desperate concert-goers tried to escape, pushing and pulling.
“I personally had a girl grab and hold on to me for dear life,” he said. “I had to calm her down because she literally thought she was about to die.”
Edward says he made a protective circle with his arms around her so she could catch her breath. He was able to pull her out of the crowd and help her to safety. As he walked out, he saw unconscious people around him, some already receiving CPR.
Another attendee, Andy Pacheco, filmed the moment concert-goer Seanna Faith, in a desperate plea for help, climbed on to a platform where a cameraman was stationed to ask him to do something about the crowd surge. She wanted the cameraman to call police or medics. But her plea for help was in vain.
Ms Faith later wrote online that she had just escaped a “sinkhole” of people as the crowding had become more intense.
She managed to pull herself and a friend out and was trying to get help for the others.
People come to these festivals to escape reality and feel like they belong in a music community. On Friday night, that sense of belonging turned deadly.