At least 14 people have been shot dead at a Batman film premiere in Denver and as many as 50 injured, according to police.
A masked gunman attacked at a premiere showing of the new Batman movie in Denver, according to reports.
Police said a suspect is in custody following the attack at the Century Aurora 16 Movie Theatre in a mall in the suburb of Aurora.
The gunman, reportedly wearing a gas mask, opened fire after the trailers had finished at a packed showing of The Dark Knight Rises and also set off a smoke or tear gas bomb.
Brenda Stuart, from 850 KOA Radio said: "This started with a midnight showing of the new Dark Knight movie and the theatres were packed that were showing this movie.
"People inside tell us they thought it was part of the movie. They heard what they thought were firecrackers, loud bangs and all of a sudden they saw the bullets flying.
"Police officers are carting the injured to the hospital in their own cars, not waiting for the ambulances."
Paul Otermat was in the cinema with his girlfriend when the gunman began firing. He said the gunman walked calmly into the cinema, threw the gas canister and began firing without saying anything.
He told Sky News: "A man walked through emergency exit. I thought it was some sort of publicity stunt for a second there and then he threw tear gas into the crowd.
"He started firing shots into the crowd. We ducked down me and my girlfriend and dragged out of the theatre to a point where we were out of the view of the shooter. We ran through the lobby and we heard more shots and ran into the parking lot and got into our car and drove off."
One Twitter user claimed her sister was inside the cinema when the shooting began.
Fuey Saechao said: "People were arguing and a Mazed was thrown, the room got Smokey and she heard like 15 gunshots. She thought it was a joke when she seen a guy with a gun until people started screaming in pain. She got down for 20 seconds and he was gone. Everyone started running out."
The gunman is reportedly aged in his early 20s and was armed with one rifle and two handguns, it was reported. Some of those injured were in an adjacent cinema and the bullets are believed to have travelled through an intervening wall.
James Cameron was in the adjacent cinema when the shooting took place. His friend suffered an asthma attack after the gas canister went off and he saw injured people as they got out of the theatre.
He told Fox News: "It has been a nightmare night. A baby girl was struck in her back. That is something that is going to stick with me for the rest of my life.
"As far as injured people, they were all over the place. It was like something from a sci-fi movie."
Sam Kiley, Defence and Security Editor, said the attack did not appear to be terror related. He said one of the victims was a six-year-old child.
He said: "Clearly on the ground everything is pointing to this being something else. Perhaps a one-off, certainly one gunman."
A makeshift hospital was set up at the mall to treat those wounded in the attack, and injured people were taken to several hospitals, according to police radio reports. Among them were four taken to a "children's hospital" and one of those killed is a six-year-old child.
Sky Sources said the Swedish Medical Center in Denver reported three injured people were being treated there. A male aged 20 and a female aged 29 were in critical condition and an 18-year-old male was in fair condition.
The Children's Hospital Colorado said it had six patients aged between six and 31.
Police also closed off surrounding parking lots as sniffer dogs were brought in to search a Hyundai vehicle for a suspected explosive device. Helicopter footage showed a bomb disposal robot approaching the white car in the parking lot.
Sky sources said the FBI were considering raising the national security alert level after the incident. ABC News reported as many as 100 agents had been assigned to the case.
Fox News initially reported there were two gunmen involved.
Police said there was no evidence a second gunman was involved, but they were investigating reports.
Witnesses described the gunman as heavily armed and reported he appeared to be wearing body armour.