At least four people died when a stage collapsed during a storm at the Indiana State Fair, where country act Sugarland was set to perform.
Some 40 people were injured in the incident at the fairgrounds in Indianapolis.
Strong winds caused the stage rigging for the outdoor concert to collapse, trapping and injuring concert-goers.
No one was performing at the time, witnesses said. The opening act had finished, and the crowd was waiting for Sugarland to take the stage.
Indiana State Police spokesman David Bursten said that a “strong gust of wind upset the rigging above the stage at the Indiana State Fairgrounds and caused a collapse of the structure over the stage”.
Mr Bursten said the injuries ranged from “very serious to cuts and scrapes”.
Emergency crews were called to the scene, and workers were setting up a command centre to tend to those who were injured.
The collapse came as many of the concert-goers, wary of the change in the weather, were trying to leave the grandstand.
The stage rigging fell onto an area where some fans were seated.
“It was like it was in slow motion,” concert-goer Amy Weathers told the Star. “You couldn’t believe it was actually happening.”