A masked, black-clad gunman in full body armour opened fire at a packed US movie theatre showing the new Batman film Friday in a bloodbath which left at least 12 dead and nearly 60 injured.
Chaos erupted when the gunman tossed two tear gas or smoke grenades into the audience at a midnight showing of "The Dark Knight Rises," then gunned down terrified patrons as they scrambled for the exits.
Police arrested the suspect - identified as 24-year-old James Holmes - without encountering resistance by his car at the rear of the theater, in Aurora, a suburb of the city of Denver, in Colorado.
Holmes, who reportedly attended the University of Colorado medical school until last month, had no criminal record aside from a citation for speeding in October 2011, according to police.
His nearby apartment was found to be booby-trapped with a sophisticated arsenal of incendiary and chemical devices, which police chief Dan Oates said could take days to defuse.
"This is the act ... of a very deranged mind," said Colorado governor John Hickenlooper at a press conference, where Oates revealed that at least three weapons were used in the attack, which injured 59, many critically.
Witnesses described chaos chillingly similar to that depicted in the Batman films, in which maniacal villains terrorize Gotham City.
"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," said 17-year-old Tanner Coon.
Cinemas in New York tightened security at Batman showings, and the AMC theatre chain announced a ban on face masks and fake weapons - several people wore costumes in Aurora, possibly helping Holmes to blend in with the melee.
The French premiere of the film in Paris was cancelled.
Holmes' parents, who live in San Diego, were shocked but "fully cooperating with law enforcement officials," said a San Diego Police Department spokeswoman, Andra Brown.
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 sombre crowd in Florida.
"The people we lost in Aurora loved and they were loved," Obama said.
As with previous such shootings - all too regular in the US - lobby groups and some political leaders called 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.
Obama ordered US flags to be flown at half-mast for six days.
Oates, the Aurora police chief, said the suspect set off two devices to distract the crowd and opened fire with an AR-15 assault rifle, a Remington shotgun and a Glock automatic pistol.
At least two of the weapons were purchased legally, according to the gun seller which sold them to Holmes, Bass Pro Shops.
"Based on the records we have reviewed, personnel in our Denver store correctly and fully followed all federal requirements with respect to the sale of one shotgun and one handgun," it said in a statement.
He was dressed all in black, with a ballistic helmet, a ballistic vest, a throat protector, a gas mask and black gloves, he said.
Ten people were killed at the scene and two more died of their wounds, police said. At least three of the wounded were US military members. A local children's hospital reported six young victims, the youngest was aged only six.
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," he said. "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 told them about the explosives at his residence, and the apartment complex where he lived - including five separate blocks - had been evacuated and was being searched.
Officers used a fire ladder to enter the building through a window.
"Our investigation determined that his apartment is booby-trapped with various chemical devices and apparent trip wires. We have an active and difficult scene there," said Oates.
Aurora is barely 20 miles (32 kilometres) from the scene of the 1999 Columbine High School massacre, in which two students shot dead 13 people before committing suicide.