Twelve people were killed and at least 38 wounded, police confirmed Friday, in a shooting at the premiere of the new Batman film in Aurora, Colorado, outside of Denver.
A 24-year-old suspect was arrested, and police were searching a flat after he spoke about possible explosives at his residence.
The attacker opened fire in a theatre at the Aurora Mall in the city, where The Dark Knight Rises was playing.
Among the injured was a 3-month-old infant, according to 9News Denver.
Federal authorities named the suspect behind the attack as James Holmes, born on September 13, 1987, according to broadcaster CNN. He had no previous run-ins with the law except traffic tickets, 9News Denver reported.
FBI officials say that Holmes entered the cinema through an exit door.
Witnesses were quoted as saying that Holmes was dressed in black wore a bullet-proof vest and riot mask. He was carrying four weapons, including an assault rifle, a shotgun and two handguns, according to CNN.
"Witnesses tell us he released some sort of canister," Oates told the Denver Post. "They heard a hissing sound and some gas emerged and the gunman opened fire."
William Washington told CNN that many initially thought the gunshots were part of the movie or fireworks.
The gunman's motive remains unclear. The incident occurred just after midnight (0600 GMT).
US President Barack Obama was given the news at 5:26 am (0926 GMT). He released a statement saying he was "shocked and saddened by the horrific and tragic shooting."
"We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded," the statement said. A planned presidential campaign stop in Florida Friday was cancelled.
On his website, Republican presidential candidate Mitt Romney also provided a statement saying he was "saddened by the news of the senseless violence that took the lives of 15 people in Colorado."
"We expect that the person responsible for this terrible crime will be quickly brought to justice," the statement read.
Warner Bros. announced that the Paris premiere for the film had been postponed. And in New York, authorities said more security would be in place at theatres where the film is showing.
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