At least 12 people have been killed and 50 wounded, after a gunman wearing a gas mask opened fire into a crowded auditorium that had come to see the new Batman film, near Denver in Colorado.

Another two victims are reported to be in a “critical condition.”

The gun man has been named as 24-year-old James Holmes, who also told police that he had booby-trapped in his flat in Aurora, Denver.

Suspect: James Holmes

Witnesses said a man wearing a gas mask opened fire after tossing a tear gas canister in the auditorium as movie-goers watched The Dark Knight Rises in the city of Aurora, at around midnight. He opened fire during an action scene in the summer blockbuster. There was chaos as movie-goers, some dressed in costume as heroes and villains, fled.

Ten people were killed at the cinema and two others died later in hospital of their wounds.

The gunman was arrested in a car park nearby in possession of a rifle, handgun and knife.

Another gun was recovered from the theatre.

Police revised down the death toll from 14 earlier. They said about 50 people had been shot, including the deceased.

Police have described the explosives in the suspect’s apartment as “very sophisticated.”

Police say the suspect had been a student at the University of Colorado School of  Medicine in Denver until he dropped out last month.

In a statement from the San Diego family home, the Holmes family, Robert and Arlene, said they were very shocked by what had happened and that their “hearts went out” to the victims.