At least four people were killed and 40 injured when a stage structure collapsed at the Indiana State Fairgrounds, local officials said early Sunday. The tragedy occurred late Saturday when a wind gust brought down the stage rigging prepared for a country music concert shortly before 9:00 pm (0100 GMT Sunday), police said. WTHR television put the number of injured at about 46. Nobody was performing at the stage when the tragedy occurred. Witnesses said the opening act had finished, and the crowd was waiting for the country music band Sugarland to begin its concert. "All of the sudden there was a second gust and the stage started to twist slowly. The grandstand started to shake. Everyone started to run instantly to try and get out," eyewitness Jason Scofield told WTHR. Emergency crews and fans quickly converged on the collapsed stage and worked to free those who were trapped, the Indianapolis Star newspapers reported. The wounded were being treated at Indianapolis area hospitals. "The response was excellent not just from first responders but from participants, people that were attending the event," said Frank Straub, of the local Department of Public Safety. "When the structure came down, people at the event worked with emergency providers tried to lift the structure to help people underneath." The group Sugarland expressed its condolences to the families of the victims through a Twitter message. "We are praying for our fans, and the people of Indianapolis, the groups said. "We hope you'll join us. They need your strength.