THOMAS PEIPERT, Associated Press Writer

People gather outside the Century 16 movie theatre in Aurora, Colo., at the scene of a mass shooting early Friday morning, July 20, 2012. Police Chief Dan Oates says 14 people are dead following the shooting at the suburban Denver movie theater. He says 50 others were injured when gunfire erupted early Friday at the Aurora theater. Oates says a gunman appeared at the front of one of the Century 16 theaters. (AP Photo/The Denver Post, Karl Gehring)

AURORA, Colorado (AP) — A gunman wearing a gas mask set off an unknown gas and fired into a crowded Colorado movie theater at a midnight opening of the latest Batman movie, killing at least 12 people and injuring at least 50, authorities said.

It was the worst mass shooting in the U.S. since the 2007 shooting on the Virginia Tech campus, and it immediately brought memories of the massacre at nearby Columbine High School in 1999, where two students opened fire and killed 12 classmates and a teacher.

Some moviegoers said they thought Friday’s attack was part of “The Dark Knight Rises,” one of the most highly anticipated films of the summer. Then they saw a silhouette of a person in the smoke at the front of the theater, pointing a gun at the crowd.

“There were bullet (casings) just falling on my head. They were burning my forehead,” Jennifer Seeger said, adding that the gunman, dressed like a SWAT team member, fired steadily except when he stopped to reload.

“Every few seconds it was just boom, boom, boom,” Seeger said. “He would reload and shoot and anyone who would try to leave would just get killed.”

The shooter was arrested shortly after the attack near a car outside the multiplex theater in Aurora. Federal law enforcement officials said the suspect is 24-year-old citizen James Holmes. The officials spoke on condition of anonymity to discuss an ongoing investigation.

Authorities did not release a motive. The FBI said there was no indication the shooting was tied to any terrorist groups.

Holmes had an assault rifle, a shotgun and two pistols, a federal law enforcement official said, speaking on the condition of anonymity to discuss the ongoing investigation.

FBI agents and police discovered Holmes’ apartment was booby trapped. Authorities evacuated five buildings as they determined how to disarm flammable and explosive material.

The suspect spoke of “possible explosives in his residence,” Aurora Police Chief Dan Oates said.

Holmes was a student at the University of Colorado School of Medicine in Denver but dropped out last month, spokeswoman Jacque Montgomery said. She did not know when he started school or why he withdrew.

Some of the injured at the theater were children, with the youngest a 4-month-old baby who has been released from treatment. Victims were being treated for chemical exposure apparently related to canisters thrown by the gunman.

“The Dark Knight Rises” opened across the world Friday, but the shooting prompted officials to cancel the Paris premiere, with workers pulling down the red carpet display at a theater on the famed Champs-Elysees Avenue.

“Warner Bros. and the filmmakers are deeply saddened to learn about this shocking incident. We extend our sincere sympathies to the families and loved ones of the victims at this tragic time,” the studio said.

President Barack Obama said he was saddened by the “horrific and tragic shooting,” and he cut short campaigning to return to the White House.

Moviegoers spoke of their terror as violence erupted.

The gunman released a gas that smelled like pepper spray from a green canister with a tag on it, Seeger said.

“I thought it was showmanship. I didn’t think it was real,” she said.

Seeger said she was in the second row when the gunman pointed a gun at her face. At first, “I was just a deer in headlights. I didn’t know what to do,” she said. Then she ducked to the ground as the gunman shot people seated behind her.

She said she began crawling toward an exit when she saw a girl about 14 years old “lying lifeless on the stairs.” She saw a man with a bullet wound in his back and tried to check his pulse, but “I had to go. I was going to get shot.”

Witness Shayla Roeder said she saw a young teenage girl on the ground bleeding outside the theater. “She just had this horrible look in her eyes …. We made eye contact and I could tell she was not all right,” Roeder said.

Aurora is home to a large Defense Department satellite intelligence operation at Buckley Air Force Base.

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Associated Press writers Kristen Wyatt, Steven K. Paulson, P. Solomon Banda, Ivan Moreno and Mead Gruver in Aurora, Dan Elliott and Colleen Slevin in Denver, Alex Katz in New York and Eileen Sullivan in Washington contributed to this report.

Copyright 2012 The Associated Press.