Bomb squad experts will try again Saturday to enter a gunman's booby-trapped apartment, hours after a late-night vigil for the 70 victims of a massacre in a packed cinema, including 12 dead.

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.

Bomb squad experts had been trying for much of the day on Friday to gain entrance to the apartment since shortly after the shooting in the town of Aurora, just outside Denver, Colorado.

But they gave up shortly before sundown, and said they would resume again on Saturday, when they hope to make a breakthrough that could also reveal clues as the motives of 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 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 the alleged gunman 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, named as James Holmes, 24, burst into a movie theater barely 20 minutes into the midnight screening of "The Dark Knight Rises," 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 Aurora police chief Dan 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, "The Dark Knight Rises," 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.

"Nearly everyone was shot," he said, adding that a "handful" of those treated in hospital did not have gunshot wounds, but suffered other injuries in the mayhem.

"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," he 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.

His nearby apartment was found to be booby-trapped with a sophisticated arsenal of incendiary and chemical devices. Police used mobile ladders to try to enter the apartment through external windows.

But Oates said late Friday that the operation had been postponed until Saturday.

Witnesses described chaos chillingly similar to that depicted in the Batman films, in which maniacal villains terrorise 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.

President 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.

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.

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.