A shocking video has emerged on YouTube showing terrified cinema-goers rushing out of screen 9 at the Century 16 cinema in Aurora, Colorado, after a gunman opened fire on the audience during the midnight showing of the new Batman film, The Dark Knight Rises.

The footage also features a policeman helping a blood-covered man out of the cinema.

The FBI was considering raising the national security level across the US after the shooting rampage in which at least 12 people died and 51 were injured.

The suspected killer was identified as James Holmes, 24. He was taken into custody by police and a bomb squad was checking a Hyundai vehicle in the parking lot for explosives. Police have found explosives in the suspect's home.

President Barack Obama released a statement saying he was shocked and saddened by the shooting.

"We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded," he said. "As we do when confronted by moments of darkness and challenge, we must now come together as one American family."

A witness told Sky News that a man dressed in black, wearing a riot helmet and body armor and carrying a shotgun kicked through the theatre's doors. Other witnesses said that the man was masked.

Many of the film fans were dressed in costume, enabling the gunman to walk among them without raising suspicion.

Local police said that the smoke that filled the auditorium was believed to be teargas and not the aftermath of a bomb explosion as first reported.

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