The suspect arrested in connection with today's fatal shootings in a Colorado cinema has been named by police as local man James Holmes.
Police say the suspect's apartment is booby-trapped and they have evacuated the area.
The suspect, who was arrested near a car outside the multiplex theatre, spoke of "possible explosives in his residence," Aurora Police Chief Dan Oates said.
Mr Oates said police also checked for explosives at the theatre and secured those areas.
Earlier today, a gunman, wearing a gas mask, set off an unknown gas and fired into the crowded cinema at a midnight opening of the latest Batman film, killing at least 12 people and injuring around 50.
Some of the injured were children, with the youngest a four-month-old baby who has been released from treatment.
There was no immediate word of a motive.
FBI spokesman Jason Pack said there was no indication in the investigation so far of any connection to terror groups.
Police Chief Oates said there was no evidence of other attackers.
"A gas mask, rifle, handgun at least one additional weapon (were) found," he said.
It was the worst mass shooting in the US since the 2007 shooting on the Virginia Tech campus, and it 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.
Victims were being treated for chemical exposure apparently related to canisters thrown by the gunman.
Aurora police spokesman Frank Fania told ABC he did not know yet if all the injuries were gunshot wounds. He said some might have been caused by other things, such as shrapnel.
President Barack Obama said he was saddened by the "horrific and tragic shooting," and he cut short campaigning to return to the White House.
Some movie-goers said they thought the attack was part of the show. 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," she said. "He would reload and shoot and anyone who would try to leave would just get killed."
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 theatre. "She just had this horrible look in her eyes …. We made eye contact and I could tell she was not all right," Roeder said.
Aurora is home to a large Defence Department satellite intelligence operation at Buckley Air Force Base.
'The Dark Knight Rises' is one of the most highly anticipated films of the summer. The movie opened across the world today, but the shooting prompted officials to cancel the Paris premiere, with workers pulling down the red carpet display at a theatre on the famed Champs-Elysees Avenue.
"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.