The shooting took place at the premiere of the new Batman movie at the Century Theater in the Aurora Mall.
Reports from local radio describe a man in a gas mask opening fire in the cinema theatre and setting off what may have been a tear gas bomb.
It is understood the two gunmen, who were said to be wearing body armour, attacked separate midnight screenings of The Dark Knight Rises within the same cinema complex.
Up to 50 people are said to have been wounded. One of two suspected gunmen has been taken into police custody. The search continues to apprehend the other suspect.
Brenda Stuart, of 850 KOA Radio, said the victims were shot dead but that there may also have been an explosion.
Police are currently searching for a second explosive device at the cinema.
The gunmen are thought to have launched their attack just as trailers before the film had had finished. They launched tear gas and then in the confusion opened fire on movie fans.
It was reported that one of the gunmen fled the scene in a vehicle with tinted windows.
The attack happened in the Denver suburb of Aurora.
Ms Stuart told Sky News: ‘A lot of people thought the gunshots were part of the movie. Then bullets started flying.’
Aurora police spokeswoman Cassidee Carlson said: ‘The scene is still very active and we have little information for release at this time.’
Nicole Williams, a spokeswoman at the Swedish Medical Centre in the town, said two people injured at the cinema had arrived at the hospital in critical condition.
Eyewitness Paul Otermat, who was in the cinema where one of the attacks took place, said the gunman came through the emergency exit, threw a tear gas canister and then opened fire.
‘There were children at that show,’ he told Sky News. ‘It was really crazy. He just walked in pretty casually. He just started firing. We waited til there was a lull in the firing, then we crawled out of there.
‘A lot of people just dropped to the floor, a lot of people made a mad dash to the exits.’