A gunman in a gas mask hurled a gas canister and opened fire in a sold-out Colorado theatre during a midnight showing of the new Batman movie on Friday, killing 12 people and injuring 59 in one of the deadliest mass shootings in recent U.S. history.

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 theatre, 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,” Ms. 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, and law enforcement officials identified him as 24-year-old citizen and neuroscience graduate student James Holmes.

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

The shooting 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. Friday’s attack was the worst mass shooting in the U.S. since an Army psychiatrist killed 13 soldiers and civilians at Fort Hood, Texas, in 2009.

In New York City, Police Commissioner Raymond Kelly said: “It clearly looks like a deranged individual. He has his hair painted red. He said he was the Joker, obviously the enemy of Batman.”

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

While some witnesses said the gunman entered through a side-door emergency exit at the front of the theater, a federal law enforcement official said the suspect bought a ticket and went into the theater as part of the crowd. He is believed to have propped open an exit door as the movie was playing, put on protective ballistic gear and opened fire. The official spoke on the condition of anonymity to discuss the ongoing investigation.

Authorities said 10 bodies were still in the theatre on Friday afternoon. Some of the injured 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 Pentagon said some military members were either killed or wounded. Aurora is home to a large Defence Department satellite intelligence operation at Buckley Air Force Base.

Police released a written 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.”

A man who lives next door to the family said Holmes seemed to be shy. Tom Mai said the mother told him Holmes couldn’t find a job after earning a master’s degree from a public university in California.

“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. Two police officers were stationed outside the AMC theater in New York’s Times Square.

“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.

Britain’s Queen Elizabeth II sent a message of sympathy on Friday to Mr. Obama. “I was saddened to learn of the tragic loss of life earlier today in Denver, Colorado. Prince Philip joins me in extending our heartfelt sympathy to you and the people of America at this time,” the British monarch said in a note sent to the White House.

In Washington, the Department of Homeland Security held a conference call with officials from the commercial, entertainment and shopping mall industries to discuss what security measures they could take in the future.

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, Ms. Seeger said.

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

Ms. 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,” Ms. Roeder said.

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