Hundreds of mourners held candles in Aurora, many sobbing and hugging each other
Bomb squad experts will try Saturday to enter a gunman's booby-trapped apartment, hours after a late-night vigil for the 12 killed in a US cinema massacre that also injured 70.
Hundreds of mourners held candles, many sobbing and hugging each other in an outpouring of grief for those who died when the gunman opened fire in a packed cinema showing Batman movie "The Dark Knight Rises."
The shooting drew expressions of concern from political leaders led by President Barack Obama, and revived the perennial debate about gun control in the United States.
On Saturday, Obama promised justice to the residents of Aurora, Colorado, saying: "The federal government stands ready to do everything necessary to bring whoever's responsible for this heinous crime to justice."
He said the government "will take every step possible" to ensure the safety of all Americans.
Bomb squad experts had been trying for much of the day Friday to gain entrance to the apartment since shortly after the shooting in the town of Aurora just outside Denver.
But they gave up shortly before sundown, and said they would resume again Saturday, when they hope to make a breakthrough that could also reveal clues as the motives of 24-year-old gunman James Holmes.
"It is a very vexing problem how to enter that apartment safely. I personally have never seen anything like what the pictures show us is in there. I'm a layman when it comes to bomb stuff, said Aurora police chief Dan Oates.
"I see an awful lot of wires, trip wires, jars full of ammunition, jars full of liquid. Some things that look like mortar rounds. We have a lot of challenge, to get in there safely."
Late Friday the town gathered for two vigils, including a midnight one, as it emerged that Holmes bought more than 6,000 rounds of ammunition on the Internet, and four guns, in the two months before the shootings.
The masked, black-clad shooter burst into a movie theater barely 20 minutes into the midnight screening, throwing two tear-gas type devices before opening fire.
"As far as we know, it was a pretty rapid pace of fire in that theater," said Oates, his voice shaking at times with emotion, and exhaustion after a long night and day dealing with the trauma.
In an end-of-day update, he amended slightly the number of victims of the shooting at a midnight screening of the latest Batman movie from 71 to 70. Twelve of them died, including 10 in the theater.
A local children's hospital reported six young victims, the youngest of whom was aged only six. At least three of the wounded were US military members, the Pentagon said.
Shots fired in one auditorium went through the wall and hit people in the auditorium next door. The first police were on the scene within 90 seconds, while eventually some 200 officers swarmed around the building.
"In the last 60 days, he purchased four guns at local metro gun shops and through the internet he purchased over 6,000 rounds of ammunition," the police chief said.
He added: "My understanding is that all the weapons that he possessed he possessed legally, and all the clips that he possessed, he possessed legally, and all the ammunition he possessed, he possessed legally."
Police arrested Holmes -- who was wearing full body armor and a gas mask, apparently to protect him from effects of his own tear gas -- without encountering resistance by his car at the rear of the theater.
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.
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.
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.
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.