Police in Houston, Texas opened a criminal investigation into the deaths of revelers at the Astroworld festival on Friday.
At least eight people died and hundreds more were injured after a wave of crowds on the opening night of the music event.
The victims were aged between 14 and 27 years. Some of their identities have begun to be made public.
Police are also investigating reports that someone in the public was injecting drugs into people.
The incident began around 9:15 pm on Friday (02:15 GMT on Saturday) when panic erupted as the crowd began to press towards the front of the stage during rapper Travis Scott’s headlining set.
When people started getting hurt, panic grew and the victims quickly overwhelmed rescuers on the spot, officials said. About 300 people were treated for injuries such as cuts and bruises.
The police investigation into the tragedy will involve homicide and narcotics divisions and will review videos of the scene to determine what caused the wave and why people were unable to escape.
Several concert-goers had to be resuscitated with anti-overdose drugs, including a security officer who police said appeared to have an injection mark on his neck.
“We have a report from a security officer … who was reaching out to hold or grab a citizen and felt a sting in the neck,” Houston Police Chief Troy Finner said.
“When he was examined he passed out,” he added. “He was resuscitated and the medical staff noticed a sting-like sting that you would get if someone were trying to inject themselves.”
Meanwhile, the names of some of the victims begin to emerge.
Brianna Rodriguez, 16, was a student at Heights High School in Houston. “Dancing was her passion for her and she is now dancing to the pearly gates of heaven,” her family wrote on Facebook.
The Dane Baig was 27 years old. His brother posted on Facebook that he was killed while trying to save another relative in the crush.
Rudy Peña, from Laredo, Texas, also died in the tragedy. His age has not yet been confirmed.
Franco Patino, 21, was an engineering student at Dayton University. His university confirmed his death on the local WHIO TV station.
In his first Twitter statement since the event, Travis Scott thanked the police and emergency services and said he was “committed to working together with the Houston community to heal and support families in need.”
He later 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 received eight Grammy Award nominations. He has a son – and another on the way – with socialite Kylie Jenner, who was among the 50,000 people at the festival.
He posted on Instagram that they were “broken and devastated”.
“I want to make it clear that we were not aware of any fatalities until the news came out after the show and in no world would we continue to film or perform,” he wrote.
In 2018, Travis Scott pleaded guilty to a public disorder charge after being charged with encouraging people to run to the stage at a concert in the U.S. state of Arkansas.
According to a local newspaper, he also paid nearly $ 7,000 (£ 5,186) to two people who said they were injured during the 2018 event.
The sense of belonging has become deadly
Angelica Casas, BBC News, Houston
The night was not supposed to end the way it did. Today, the streets surrounding the Astroworld festival area are still mostly closed and the venue is almost empty except for staff and a strong police presence.
Edward, a 25-year-old Houston native and longtime Travis Scott fan, attended performances on Friday and both previous Astroworld festivals.
He was right in the crowd when he, he says, things started to go wrong. As the wave got worse within 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.