Local media sources in Colorado are reporting that 10 people, including children, have been killed at the premiere of Batman movie The Dark Knight Rises in the suburbs of Denver.
Brenda Stuart, of 850 KOA Radio, said the victims were shot dead but there may have also been an explosion. It is reported that many of the victims are children.
She said police are looking for a second explosive device at the Centruy 16 movie theatre in Aurora Mall.
One suspect has been detained.
As many as 40 people are believed to have been injured in the attack.
Ms Stuart said: "The shooting started during the midnight showing at the theatre. Many people thought it was part of the movie before panic set in.
"Police are taking people to hospital in their own cars, not waiting for ambulances to arrive. There were chaotic scenes as people desperately tried to flee.""
Fox News is reporting that two people were involved in the attack . They set off tear gas, possibly in two separate theatres.
The FBI are on the scene to co-ordinate the hunt for the second man. But their efforts are being hampered by fears that a bomb may have been planted in the movie theatre parking lot.
The movie complex was sold out for the screening.
The preview trailers had just finished when a gas canister exploded, according to witnesses. It is believed the gunmen opened fire as people began to flee the theatres..