A masked attacker gunned down dozens of US moviegoers at the packed premiere of a Batman movie on Friday, killing at least 12 people and wounding almost 40.
Chaos erupted when the gunman tossed some kind of grenade into the audience at a midnight showing of "The Dark Knight Rises," then opened fire on terrified patrons as they scrambled for the exit.
Local police arrested the killer -- identified by US media as 24-year-old local man James Holmes -- at the rear of the theater, in Aurora, a suburb of the city of Denver, in Colorado.
Local media quoted a police chief as saying the suspect's home may have been booby trapped with explosives, and television footage showed officers in body armor breaking into the apartment through a window.
Cinemas in New York tightened security at Batman showings following the attack, and the French premiere of the film in Paris was cancelled.
"I saw some people start to get up. I poked my head up to see what was going on and when I did that I saw another flash and instantly put my head down as he started shooting again," 17-year-old Tanner Coon told NBC.
"I went to try to get to the exit and slipped on some blood, landed on a lady. I saw her heels. I shook her. I told her: 'We need to get up. You need to get out of here.' There was no response. So I presumed she was dead."
The suspect is thought to be a student from California. A spokeswoman for the San Diego Police Department, Andra Brown, said his family was shocked by the incident and cooperating with law enforcement.
"Their hearts go out to those involved or touched by this," she said, adding that the family's San Diego home had been put under police protection.
President Barack Obama cut short a campaign trip to Florida and returned to the White House to address the situation, although his spokesman said there appeared to have been no link to terrorism.
"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 president told a somber crowd in Florida.
"The people we lost in Aurora loved and they were loved," Obama said.
Following the shooting, Obama and his Republican rival Mitt Romney came under pressure on gun control, with lobby groups and some political leaders calling for legislation to restrict civilians' access to firearms.
"Maybe it's time that the two people who want to be president of the United States stand up and tell us what they are going to do because this is obviously a problem across the country," New York mayor Michael Bloomberg said.
Both campaigns offered sympathy for the dead, but neither promised to change federal gun law. The right to bear arms in enshrined in the second amendment to the US constitution.
"The president believes we need to take common-sense measures that protect Second Amendment rights of Americans, while ensuring that those who should not have guns under existing law do not get them," Obama's spokesman said.
"There has been progress in that regard in terms of improving the volume and quality of information in background checks," he said "But I have nothing new -- nothing additional on that for you."
In a statement, the Aurora Police Department said: "Police were told the suspect 'appeared' at the front of one of the theaters, threw some type of gas or explosive device and started shooting."
Aurora police chief Dan Oates said police had arrested an alleged gunman and there was "no evidence" of a second, after earlier reports of two shooters.
"When the suspect was arrested, police recovered one rifle, one handgun and a gas mask. A third gun is still inside the theater. The suspect ... offered no resistance when he was arrested," it added.
Ten people were killed at the scene and two more died of their wounds, police said. A local children's hospital reported six young victims, the youngest was aged only six.
"The injured and dead include a wide age range. The conditions of the injured also vary, with some suffering very serious wounds," said police.
Chris Jones, who was in the theater, 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.
"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."
Shots fired in one auditorium went through the wall and hit people in the auditorium next door. Jones said by the time he got out, police were already in the building.
Oates said the suspect claimed to have explosives at his residence and the apartment complex where he lived had been evacuated and was being searched.
Television images showed officers using a fire ladder to enter the building through a window.
Police spokesman Frank Fania told CNN the shooter was wearing body armor and armed with a rifle and two handguns, adding that he had set off some kind of smoke device in order to sow panic.
Witnesses described chaos chillingly similar to that depicted in the Batman films, in which maniacal villains terrorize Gotham City.
They said several audience members had shown up in costumes, which could have allowed the gunman to blend in with the melee and complicated the arrest.
Aurora is barely 20 miles (32 kilometers) from the scene of the 1999 Columbine High School massacre, in which two students shot dead 13 people before committing suicide.