Families of the victims have demanded justice as a criminal investigation is now under way, with Houston mayor Sylvester Turner calling the disaster a "tragedy on many different levels".

Thirteen people remained in hospital on Sunday. Details of their conditions have not been released.

A security officer was also left unconscious after apparently being injected in the neck by a concert-goer. Houston city police chief Troy Finner said his department had opened a criminal investigation by homicide and narcotics detectives, following reports that somebody in the audience had been injecting people with drugs.

Houston police and fire department officials have said their investigation will include reviewing video taken by concert promoter Live Nation, as well as dozens of clips from people at the show.

Officials also looked to review the event's security plan and various permits issued to organisers to see whether they were properly followed. In addition, investigators planned to speak with Live Nation representatives, Scott and concert-goers.

Scott, who founded the festival, said he was "devastated" following the deaths, and was working to help the families of "the ones that was lost" as well as giving his "total support" to police.

The 29-year-old star is known for his high-energy performances in which music fans are encouraged to stage dive and crowd-surf.

He stopped multiple times during his set after spotting fans in distress near the front of the stage, and asked security to help people out of the crowd.

His partner Kylie Jenner, with whom he is expecting his second child, said after the tragedy that they had been left "broken and devastated", and offered her "deepest condolences" to the families of those affected.

Sky News has contacted representatives for Scott and Live Nation for comment. A spokesperson for Drake declined to comment on the lawsuit reports.