About 50 others, including children, were wounded in the attack on the showing of "The Dark Knight Rises" in a mall in the Aurora suburb, some of whom were treated for the effects of tear gas, hospital officials said.
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Federal law enforcement officials identified the suspect in the shooting as James Holmes, a 24-year-old white American, from Aurora. He is in custody.
US President Barack Obama said he was "shocked and saddened" by the shooting: "Michelle and I are shocked and saddened by the horrific and tragic shooting in Colorado."
"My administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time," he said. "We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded."
"As we do when confronted by moments of darkness and challenge, we must now come together as one American family," he concluded.
Movie theater in Aurora, Colorado (Photo: AP)
Aurora Police said that the gunman had appeared at the front of the theater during the movie and released a canister which let out a hissing sound before gunfire erupted.
Dozens of police were at the scene and the authorities evacuated the area while they checked for any explosive devices. They said there was no evidence of a second gunman.
Colorado has suffered mass killings in the past. In 1999, two students opened fire at Columbine High School in the suburb of Littleton, near Denver, killing 12 students and a teacher.
Emergency services at the scene (Photo: AP)
Police spokesman Frank Fania said the suspect was carrying a knife, a rifle and a handgun when arrested, and one other gun was recovered from the theater. The gunman was also wearing a bulletproof vest.
"He did not resist, he did not put up a fight," Fania said.
Police said the suspect's apartment building had been evacuated and police were looking for explosives there after the suspect made statements about explosives in his residence.
Fania said police received the first call about the shooting at 12:39 am local time (06:39 GMT) and responded within "a minute or two."
Local hospitals were alerted to a "mass casualty incident." Fania told NBC News that 10 of the victims died in the theater and two died in the hospital.
A statement by Warner Bros said that the studio was "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."
AP contributed to this report
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