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Photo: The Canadian Press. All rights reserved.

A SWAT team officer stands watch near an apartment house where the suspect in a shooting at a movie theatre lived in Aurora, Colo., Friday, July 20, 2012. As many as 14 people were killed and 50 injured at a shooting at the Century 16 movie theatre early Friday during the showing of the latest Batman movie. (AP Photo/Ed Andrieski)

A gunman wearing a gas mask set off an unknown gas and fired into a crowded movie theatre 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 theatre 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, firing as he went, saying nothing.

Some of those injured are children, including a 3-month-old infant who was shot.

Police earlier said 14 people were dead, but later revised the number to at least 12. It was not immediately clear for the reason for the revision.

Witness Shayla Roeder said she saw a young teenage girl on the ground bleeding outside the theatre.

"She just had this horrible look in her eyes .... We made eye contact and I could tell she was not all right," Roeder said.

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 theatre at a mall in Aurora.

FBI spokesman Jason Pack said there's no indication 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 theatre, 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 theatre.

"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.

A large truck lettered "bomb squad" arrived near an Aurora apartment complex where the suspect is believed to have lived, about four miles from the theatre. Dozens of police squad cars, vans and other vehicles were already at the scene, along with black-clad officers carrying automatic weapons.

Police evacuated residents of the building. Oates did not say whether any explosives had been found.

Raw video taken from a cell phone at the theater (Caution, video may contain language not suitable for all audiences).