Aurora: Police say 71 people were shot in the suburban Denver movie theatre early Friday during midnight shows of the new Batman movie.
Twelve people were killed, ten of them at the theatre.
Another 59 adults and children were wounded.
Aurora Police Chief Dan Oates says there were four showings of the movie at the time and all were sold out. He did not know how many people that amounted to.
Oates says investigators are confident the gunman acted alone.
Police arrested 24-year-old James Holmes, whose apartment four miles away was booby trapped.
Oates says Holmes wore body armour, used an assault rifle, a shotgun and a Glock handgun.
He says Holmes' car was parked in back of the theatre.
Television networks cited the FBI as saying Holmes, from the town of Aurora, Colorado, scene of the shooting at a screening of the Batman movie The Dark Knight Rises, did not appear to have any known terrorism connections.
At least 12 people were killed and around 40 were wounded on Friday when a gunman opened fire at the crowded cinema premiere, police said.
It had been reported that as many as 14 were killed.
Following the arrest, police headed to the suspect’s apartment.
Local police chief Dan Oates said the suspect had claimed to have explosives at his residence and that the complex where he lived had been evacuated and was being searched.
Live television footage showed officers on a mobile ladder attempting to enter the apartment through a window.
Earlier, witnesses described a scene of chaos at the midnight debut showing of the movie, saying the gunman had set off tear gas bombs and opened fire in the packed theatre in Aurora, a suburb of Denver, Colorado.
The shooter was arrested at the rear of the theatre, police said.
“Police were told the suspect “appeared” at the front of one of the theatres, threw some type of gas or explosive device and started shooting,” said an Aurora Police Department statement.
“When the suspect was arrested, police recovered one rifle, one handgun and a gas mask. A third gun is still inside the theatre. The suspect ... offered no resistance when he was arrested,” it added.
“The injured and dead include a wide age range. The conditions of the injured also vary, with some suffering very serious wounds,” said the police statement.
Aurora police chief Dan Oates said there was “no evidence” of a second gunman, after earlier reports of two shooters.
President Barack Obama, who was in Florida, expressed shock at the shooting, after being told about it shortly before 5.30am.
“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.”
“We are committed to bringing whoever was responsible to justice. As we do when confronted by moments of darkness and challenge, we must now come together as one American family.”
Chris Jones, who was in the theatre, said the shooting began about 20 to 30 minutes into the screening.
“People just started dropping. We were on the floor trying not to get shot,” Jones told the local KMGH-TV television station.
“I thought it was just fireworks. There was smoke, then I heard ‘bam, bam, bam.’ The gunman didn’t have to stop to reload. Shots just kept coming, kept coming, kept coming.”