The violent and chaotic scene erupted about 12:30 a.m. MDT as a gunman stood at the front of one of the Century 16 theaters at the Aurora Mall where the latest Batman movie The Dark Knight Rises was playing, police said. (Top papers, sites on Batman shooting)(Tweets)
"Witnesses tell us he released some sort of canister. They heard a hissing sound and some gas emerged and the gunman opened fire," Mr Oates said at a news conference.
Police, ambulances and emergency crews swarmed on the scene after frantic calls started flooding the 911 switchboard, officials said.
Officers found the gunman near a car behind the theater.
"A gas mask, rifle, handgun at least one additional weapon (were) found inside" he said.
The suspect was taken into custody, but no name was released. Mr Oates said there's no evidence of any other attackers. There was also no immediate word of any motive.
The suspect spoke of "possible explosives in his residence. We are dealing with that potential threat," Mr Oates said, without providing details on where that was.
He said police also checked for explosives in the parking lot and theater, and secured those areas.
Witness Hayden Miller told KUSA-TV that he was inside Theater 16 and heard several shots.
"Like little explosions going on and shortly after that we heard people screaming," he told the station.
Hayden said at first he thought it was part of a louder movie next door. But then he saw "people hunched over leaving theater."
The police chief said 10 victims died at the theater and four at area hospitals.
Two people in critical condition were rushed to the nearby Swedish Medical Center, spokeswoman Nicole Williams said.
She said emergency crews told her staff to prepare for several more patients from the shooting scene.
"We're on standby to receive," she said.
Aurora is located on the southeast outskirts of Denver, about 10 miles from downtown.