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

As the smoke from the cartridge case spread, viewers in the suburban Denver movie theater saw the silhouette of a person appear near the screen, point a gun at the crowd and begin firing, apparently without saying a word.

It was one of the deadliest mass murders in recent US history.

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

Authorities did not give a motive for the attack. The FBI said there was no evidence of links to terrorist groups.

“I had shell casings falling on my head. They were burning my forehead,” Jennifer Seeger said, adding that the gunman, who was dressed like a member of a SWAT team, fired continuously, stopping only to reload. “Every few seconds it just went: boom, boom, boom,” she said. “He reloaded and fired, and anyone who tried to escape was just killed.”

According to police, a total of 71 people were shot.

Aurora Police Chief Dan Oates said the gunman was wearing a gas mask, ballistic helmet and body armor, as well as leg, groin and throat protectors. He said he was carrying an AR-15 semi-automatic military rifle, a shotgun and two pistols.

Some witnesses said the gunman entered through a side exit at the front of the theater, but a federal official who asked not to be identified said Holmes bought a ticket and went into the theater with the crowd.

The officer said Holmes then apparently propped open an exit door in the theater while the film was playing, put on ballistic protective gear and opened fire.

FBI agents and police used a fire truck and ladder truck to reach Holmes’s Aurora apartment. They placed a camera on the end of a 12-foot pole inside the apartment and discovered that the camera was booby-trapped. Authorities evacuated five buildings while they tried to figure out how to defuse the flammable and explosive material.

“I’ve never seen anything like this,” Oates said.

Some of the victims were treated for chemical poisoning, apparently caused by canisters thrown by the gunman. Among the injured was a four-month-old baby who was treated at the hospital and released.

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

Police released a statement from Holmes’ family: “Our deepest condolences go out to the victims of this tragedy and to the families and friends of those affected.”

The film opened in theaters worldwide on Friday, with midnight screenings in the United States. Because of the shooting, the red carpet premiere in Paris was canceled, and workers tore down the screen at a cinema on the Champs-Elysees.

Across America, police and some theaters increased security for daytime screenings of the film, but many fans waiting in line said they were not concerned for their safety.

President Barack Obama expressed his sadness over the “cruel and tragic shooting” and promised that his administration was committed to “bringing those responsible to justice, keeping our citizens safe and caring for those injured.”

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

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

The attack on Friday began shortly after midnight at the multiplex cinema and viewers said they believed it was part of the film or some kind of stunt related to the movie.

The film contains several scenes of public chaos – a hallmark of superhero films. In one scene, the villain Bane leads an attack on the stock exchange, and in another he goes on a shooting and bombing spree in a packed football stadium.

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

Seeger said she was standing in the second row, about three feet away from the shooter, when he pointed a gun in her face. At first, “I was like a deer in the headlights. I didn’t know what to do,” she said. Then she ducked to the ground as the shooter fired at the people sitting behind her.

She said she crawled toward an exit when she saw a girl of about 14 “lying lifeless on the stairs.” She saw a man with a gunshot wound in his back and tried to feel his pulse, but “I had to go. I would have been shot.”

Shayla Roeder said she saw a young girl bleeding on the ground outside the theater. “She had this horrible look in her eyes. … We made eye contact and I knew she wasn’t OK,” Roeder said.

Police, ambulances and rescue crews flocked to the scene after 911 dispatchers were flooded with panicked calls. Officers came running and told people to leave the theater, Salina Jordan told the Denver Post. She said some police officers were carrying and dragging bodies.

Hayden Miller told KUSA-TV that he heard several gunshots. “There were small explosions and shortly after that we heard people screaming,” he told the station. Hayden said he initially thought it was a louder movie next door. But then he saw “people hunched over leaving the theater.”