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

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 1:30 a.m. Butler time at the multiplex theater at a mall in Aurora.

A Colorado police spokesman this morning revised the death toll to at least 12. An earlier report stated that 14 were killed.

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. Two students 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.