Witnesses say they heard a series of explosions and up to 20 gunshots after the scene grew chaotic. About 100 witnesses were taken to a local high school to be questioned by police.
Police, ambulances and emergency crews swarmed on the Aurora, Colo., theater after frantic 911 calls around 12:30 a.m. local time, officials said.
The suspect reportedly fired shots inside the theater and fled to the parking lot and was confronted by police already at the theater for crowd control.
James Cameron, who was in an adjacent theater, said he hear commotion and screaming. People seated around him began coughing and having difficulty breathing.