Passengers suffered a frightening ride when two carriages on a roller coaster at Alton Towers collided, causing four teenagers to be injured.

According to BBC News, one of the carriages had no one inside the Smiler ride while the other was filled with sixteen passengers. Witnesses who heard the “loud crash” reported that many people were “screaming and shouting” for help.

The collision happened at 2:09 pm in England (9:09 am EST) and first aid staff were immediately on scene as emergency responders created a platform that would help them to rescue the people on the ride.

One witness reported that the coach that was empty was apparently “stuck” when the other coach crashed into it. “There was a big crash, and I think there were quite a few people that were angry because they said that they had told the staff that the coach before had stopped but they hadn’t actually called through.”

The Guardian reported the four passengers had leg injuries that required medical attention. The roller coaster opened in May 2013 and has experienced technical issues in the past. Witnesses also reported that the ride had issues earlier in the day before the crash occurred.

During a press launch after its opening, sixteen individuals were rescued after the ride broke down. In the same year, wheels fell off the ride and hit four bystanders, causing the ride to be shut down.