AURORA, Colo. —  A gunman wearing a gas mask and black SWAT gear hurled a gas canister inside a crowded movie theater during a midnight showing of the new Batman movie Friday and then opened fire, killing 12 people and wounding nearly 60 others in an attack so bizarre that some moviegoers at first thought they were watching Hollywood special effects.

As smoke from the canister spread, audience members watching “The Dark Knight Rises” at the suburban Denver theater saw the silhouette of a person materialize near the screen, point a gun at the crowd and begin shooting, apparently without a word.

It was one of the deadliest mass shootings in recent U.S. history.

The suspected gunman, identified as James Holmes, a 24-year-old graduate student in neuroscience at the University of Colorado-Denver, was arrested near a car behind the theater.

Authorities gave no motive for the attack. The FBI said there was no indication of ties to any terrorist groups.

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

Police said 71 people in all were shot.

Aurora Police Chief Dan Oates said the gunman wore a gas mask, a ballistic helmet and vest as well as leg, groin and throat protectors. He said he had an AR-15 military-style, semi-automatic rifle, a shotgun and two pistols.

While some witnesses said the gunman entered through a side-door emergency exit at the front of the theater, a federal law enforcement official, speaking on condition of anonymity, said Holmes bought a ticket and went into the theater as part of the crowd.

The official said Holmes then apparently propped open an exit door in the theater as the movie was playing, donned the protective ballistic gear and opened fire.

FBI agents and police used a hook-and-ladder fire truck to reach Holmes’ apartment in Aurora. They put a camera at the end of a 12-foot pole inside the apartment and discovered the unit was booby-trapped. Authorities evacuated five buildings as they tried to figure how to disarm the flammable and explosive material.

“It’s something I’ve never seen before,” Oates said.

Some of the victims were treated for chemical exposure apparently related to canisters thrown by the gunman. Those hurt included a 4-month-old baby, who was treated at a hospital and released.

Holmes enrolled in a Ph.D. program in neuroscience a year ago but was in the process of withdrawing at the time of the shooting, said University of Colorado-Denver spokeswoman Jacque Montgomery said.

Police released a statement from Holmes’ family: “Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved.”

The movie opened across the world Friday with midnight showings in the U.S. The shooting prompted officials to cancel the red-carpet premiere in Paris, with workers pulling down the display at a theater on the Champs-Elysees.

Around the U.S., police and some movie theaters stepped up security for daytime showings of the movie, though many fans waiting in line said they were not worried about their safety.

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.”

It was the worst mass shooting in the U.S. since the Nov. 5, 2009, attack at Fort Hood, Texas. An Army psychiatrist was charged with killing 13 soldiers and civilians and wounding more than two dozen others.

In Colorado, it was the deadliest since the Columbine High School massacre on April 20, 1999, when two students opened fire in the Denver suburb of Littleton, killing 12 classmates and a teacher and wounding 26 others before killing themselves. Columbine High is about 12 miles from the theater.

Friday’s attack began shortly after midnight at the multiplex theater, and audience members said they thought it was part of the movie, or some kind of stunt associated with it.

The film has several scenes of public mayhem — a hallmark of superhero movies. In one scene, the villain Bane leads an attack on the stock exchange and, in another, leads a shooting and bombing rampage on a packed football stadium.

The gunman released a gas that smelled like pepper spray from a green canister, Seeger said. “I thought it was showmanship. I didn’t think it was real,” she said.

Seeger said she was in the second row, about four feet from the gunman, when he 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 of about 14 “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.”

Shayla Roeder said she saw a 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.

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

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.”

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Associated Press writers Kristen Wyatt, Steven K. Paulson, Ivan Moreno and Mead Gruver in Aurora, Dan Elliott and Colleen Slevin in Denver and Alicia A. Caldwell and Eileen Sullivan in Washington contributed to this report.