At least 12 people have been killed and 59 wounded in a shooting at a midnight cinema showing of the new Batman film near Denver, Colorado.
Witnesses said a man wearing a gas mask opened fire after tossing a tear gas canister in the auditorium as movie-goers watched The Dark Knight Rises.
A 24-year-old former neuroscience student was arrested in a car park outside the theatre in Aurora city.
He was armed with a rifle, a shotgun and two pistols, said officials.
Police revised down the death toll from 14 earlier. They said 71 people, including the deceased, had been shot in the incident at the Century 16 multiplex cinema.
FBI sources named the gunman as Aurora resident James Holmes, a white American. They said no terrorism link had been established.
Police said he had been in the process of withdrawing from his PhD studies at the University of Colorado-Denver.
In a written statement, the Holmes family in San Diego said they were co-operating with police in San Diego and Aurora, and asked for the media to respect their privacy.
"Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved."
US President Barack Obama cut short a campaign trip to Florida and returned to the White House to address the situation.
Acting on information from the suspect, the authorities said they had discovered that his home in the north of Aurora has been booby-trapped. Incendiary devices with chemical elements and trip wires are in the property, police said.
The apartment complex and five buildings nearby were evacuated as FBI agents and police used a fire engine ladder to reach the property, and put a camera on the end of pole to look inside.
Police chief Dan Oates said the massacre was "a horrific event". "The shooting apparently went on for some time," he added.
Cinemas in New York tightened security at Batman showings following the attack, and the French premiere of the film in Paris was cancelled.
Witnesses said the gunman opened fire during an action scene in the summer blockbuster, at about 00:30 local time (06:30 GMT).
There was chaos as movie-goers, some dressed in costume as heroes and villains, fled.
Ten people were killed at the cinema and two others died later in hospital of their wounds.
Many ambulances attended the scene.
Scores of people, some in a critical condition, were taken to four hospitals.
The casualties included a four-month-old baby, who was released from hospital after treatment, and a six-year-old child.
The Pentagon said members of the military were among the casualties.
'Firing at random'
One witness said the gunman had been "slowly making his way up the stairs and just firing - picking random people".
Another witness, Chayyiel Jackson, told the BBC: "During the first action scene in the movie, on the right side a dude came in all blacked out, with a black mask. At first we thought it was part of the movie event.
"He threw tear gas across the crowd and after that people started to panic. He pulled out a rifle and started shooting.
"I was wondering if this was real life or a dream."
At least one person in an adjacent auditorium was injured when a bullet went through the wall, police said.
Another eyewitness, identified only as Pam, told the BBC: "He fired a canister into the air. It shot it right into the air, then I started to hear the bang, bang, bang of a gun.
"I crawled my way through the row. Luckily the exit was close to where I was sitting so I got out pretty fast. I heard more shots as I left."
Another eyewitness, Salina Jordan, 19, told the Denver Post she had seen one girl shot in the cheek, and a girl who appeared to be about nine years old with a gunshot wound to the stomach.
New York Police Commissioner Raymond Kelly - who previously worked with Aurora police chief Dan Oates and spoke with him on Friday - said in a news conference said the suspect "had his hair painted red" and claimed to be The Joker, a villain in the last Batman film.
Aurora officials declined to comment on Mr Kelly's statement.
A witness in the next-door theatre told 9News he initially thought live special effects had been laid on by the cinema after hearing loud bangs and seeing smoke.
President Obama told a sombre crowd in Florida: "Such violence, such evil, is senseless. But, while we will never know fully what causes somebody to take the live of another, we do know what makes life worth living.
"The people we lost in Aurora loved and they were loved."