A GUNMAN wearing a gas mask has killed 14 people when he opened fire at a Colorado cinema on the opening night of the latest Batman movie.
At least 50 others were injured.
The gunman was aged in his early twenties, Aurora police spokesman Frank Fania said.
FBI spokesman Jason Pack said there was no indication so far of any connection to terrorism.
President Barack Obama said he was “shocked and saddened” by the “horrific and tragic” shooting.
It was apparently the worst mass shooting in the US since the 2007 shooting on the Virginia Tech campus.
The gunman stood at the front of the cinema and fired into the crowd. He was later taken into custody near a car behind the cinema.
“Witnesses tell us he released some sort of canister. They heard a hissing sound, and some gas emerged and the gunman opened fire,” Aurora Police Chief Dan Oates told a news conference.
Mr Fania told ABC that investigators do not believe anyone else was involved.
Emergency crews rushed to the scene after frantic calls started flooding the switchboard.
Officers ran in and told people to leave the cinema, Salina Jordan told the Denver Post newspaper. She said some police were carrying and dragging bodies.
“A gas mask, rifle, handgun at least one additional weapon (were) found,” Mr Oates said.
The suspect’s name was not released. There was 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
Police were at the Denver-area apartment and had evacuated other residents of the building. Mr Oates did not say whether explosives had been found.
He said police also checked for explosives at the Century 16 theatre and secured those areas.
The police chief said 10 victims died at the cinema and four at area hospitals. At least 24 people were being treated at hospitals.
Moviegoers spoke of their terror as violence erupted and people around them were shot.
Benjamin Fernandez, 30, told the Post he heard a series of explosions. He said people ran from the theatre and there were gunshots as police shouted “Get down!”
Ms Jordan told the paper that one girl was struck in the cheek, and others were struck in the stomach, including a girl who looked to be around nine years old.
She said it sounded like firecrackers until someone ran into the neighbouring theatre yelling, “They’re shooting out here!”
Hayden Miller told KUSA-TV he heard several shots.
“Like little explosions going on, and shortly after that we heard people screaming,” he told the station.
Mr Hayden said at first he thought it was part of a louder film next door. But then he saw “people hunched over leaving the theatre.”
KUSA reported that some victims were being treated for chemical exposure, related apparently to the canister thrown by gunman.
Eleven people were being treated at the Medical Centre of Aurora for gunshots and ranged from minor to critical condition. Two others walked in to be treated for tear gas contamination. Denver Health had seven victims - one in critical and the rest in fair condition.
The youngest victim reported was a six-year-old being treated at Children’s Hospital Colorado, where a total of six victims were taken. Their condition was not known.
Two people in critical condition were rushed to nearby Swedish Medical Centre, spokeswoman Nicole Williams said.
Aurora is home to a large Defence Department satellite intelligence operation at Buckley Air Force Base.
Some movie-goers said they thought the attack was part of the show. Then they saw a silhouette of a person in the smoke at the front of the cinema, pointing a gun at the crowd.
“I told my friend, ‘We’ve got to get out of here,’ but then he shot people trying to go out the exits,” Jennifer Seeger told NBC. She said the shooter made his way up the aisle, shooting as he went, saying nothing.
Some of the injured were children, with the youngest a three-month-old baby.