UPDATE, 8:54 a.m.: Authorities say 12 people were killed and at least 50 others were injured. Police originally said 14 people died in the attack but later revised the death toll.

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A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theater in suburban Denver at a midnight opening of the Batman movie "The Dark Knight Rises," killing 12 people and injuring at least 50 others, authorities said.

Moviegoers didn't know what was happening and some thought the attack was part of the show. Then they saw a silhouette of a person in the smoke at the front of the theater near the screen, 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's "Today." She said the shooter made his way up the aisle, shooting as he went, saying nothing.

Some of those injured are children, and victims are being treated for chemical exposure apparently related to canisters thrown by the gunman. The shooter, a man in his 20s, was arrested shortly after the attack at 12:30 a.m. MDT at the multiplex theater at a mall in Aurora.

FBI spokesman Jason Pack said there's no indication in the investigation so far of any connection to terror groups.

It was the worst mass shooting in Colorado since the Columbine High School massacre on April 20, 1999. Students Eric Harris, 18, and Dylan Klebold, 17, opened fire at the school in the Denver suburb of Littleton, about 15 miles west of Aurora, killing 12 classmates and a teacher and wounding 26 others before killing themselves in the school's library.

Aurora police spokesman Frank Fania on ABC's "Good Morning America" said he didn't know yet if all the injuries were gunshot wounds. He said some might have been caused by other things such as shrapnel.

Police, ambulances and emergency crews swarmed on the scene after frantic calls started flooding the 911 switchboard, officials said.

Officers came running in and telling people to leave the theater, Salina Jordan told the Denver Post. She said some police were carrying and dragging bodies.

Officers later 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. 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," Oates said

Police were at the Denver-area apartment and had evacuated other residents of the building. Oates did not say whether any explosives had been found.

He said police also checked for explosives in the parking lot and at the Century 16 theater and secured those areas.

President Barack Obama said he was saddened by the "horrific and tragic shooting," pledging that his administration was "committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded."

Moviegoers spoke of their terror as violence erupted and people around them fell victim.

Benjamin Fernandez, 30, told the Post that he heard a series of explosions. He said that people ran from the theater and there were gunshots as police shouted "get down!"

Frenandez said he saw people falling, including one young girl.

Jordan told the paper that one girl was struck in cheek, others in stomach including a girl who looked to be around 9-years-old.

Jordan said it sounded like firecrackers until someone ran into Theater 8 yelling "they're shooting out here!"

Hayden Miller told KUSA-TV that he 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.

At least 24 people were being treated at Denver area hospitals.

KUSA reported that some hospitalized victims were being treated for chemical exposure, related apparently to canister thrown by gunman.

Eleven people were being treated at the Medical Center 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 6-year-old being treated at Children's Hospital Colorado, where a total of six victims were taken. Their condition wasn't known.

Two people in critical condition were rushed to nearby Swedish Medical Center, spokeswoman Nicole Williams said.

Aurora is on Denver's east side and is Colorado's third-largest city with 327,000 residents. It is home to a large Defense Department satellite intelligence operation at Buckley Air Force Base, as well as The Children's Hospital, the University of Colorado Hospital and a future Veterans Affairs hospital.