US President Barack Obama has said he is "shocked and saddened" by the deadly mass shooting at a Colorado cinema screening of the The Dark Knight Rises.
Federal law enforcement officials confirmed the lone suspect being detained by police was 24-year-old James Holmes. Police have cordoned off his apartment - a three block area.
A California woman who identified herself as the mother of James Holmes, the 24-year-old man federal authorities said is the suspect in a mass shooting in an Aurora, Colorado movie theater, told ABC News her son was likely the culprit, saying, "You have the right person."
The woman, contacted at her home in San Diego, spoke briefly with ABC News and immediately expressed concern her son may be involved in the shooting.
"You have the right person," she said, apparently speaking on gut instinct. "I need to call the police... I need to fly out to Colorado."
President Obama urged the nation to "come together as one American family".
He described the shooting as "horrific and tragic" and said his administration will do everything it can to support the people of Aurora.
He was in Florida at the time of the murder spree. The White House said Mr Obama was informed of the shooting by his Homeland Security Adviser, John Brennan.
Police have revised the death toll down to 12. Earlier reports feared 14 people had been murdered.
t was the worst mass shooting in Colorado since the Columbine High School massacre in 1999, where two students opened fire and killed 12 classmates and a teacher.
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.
"I told my friend, 'We've got to get out of here,' but then he shot people trying to go out the exits," Jennifer Seeger told NBC. She said the shooter made his way up the aisle, shooting as he went, saying nothing.
Some of the injured were children, with the youngest a three-month-old baby. Victims were being treated for chemical exposure apparently related to canisters thrown by the gunman.
Local reporter Justin Jones said the gunman was wearing body armour and a gas mask.
"The attacker shot a baby at point blank range," he said.
Another eyewitness, James Cameron, said the baby girl was shot in the back.
Aurora Police chief Dan Oates said one suspected gunman had been taken into custody - and police were not looking for a second suspect at this stage.
"One of our highest priorities will be dealing with the famlilies of the victims," he said.
"Ten people died in the theatre and two died in hosital as a result of their injuries."
Witness Hayden Miller told KUSA-TV that he was inside Theatre 16 in a suburb of Denver when he heard several shots.
"Like little explosions going on and shortly after that we heard people screaming," he told the station.
Hayden said at first he thought it was part of a louder movie next door. But then he saw "people hunched over leaving theatre".
Brenda Stuart, of 850 KOA Radio, said the victims were shot dead but there may have also been an explosion. It is reported that many of the victims are children.
She said police are looking for a second explosive device at the Centruy 16 movie theatre in Aurora Mall.
As many as 50 people are believed to have been injured in the attack.
Ms Stuart said: "The shooting started during the midnight showing at the theatre. Many people thought it was part of the movie before panic set in.
"Police are taking people to hospital in their own cars, not waiting for ambulances to arrive. There were chaotic scenes as people desperately tried to flee.""
Fox News had earlier reported that two people were involved in the attack .
The FBI are on the scene to co-ordinate the investigation but their efforts are being hampered by fears that a bomb may have been planted in the movie theatre parking lot.
The movie complex was sold out for the screening.
The preview trailers had just finished when a gas canister exploded, according to witnesses. It is believed the gunman opened fire as people began to flee the theatres..