US President Barack Obama will visit Colorado on Sunday to meet victims of the Batman theater massacre, a day after police entered the gunman's apartment which had been rigged with booby traps.

The visit also comes after authorities revealed the names of the 12 people who died in Friday's shootings -- including a six-year-old girl whose mother was also injured and two US servicemen.

The White House said Obama would meet with families of the victims and local officials in Aurora, near Denver where James Holmes, 24, is said to have opened fire during a packed midnight screening of "The Dark Knight Rises."

The actor who plays the caped crusader in the film, the last in the Batman trilogy, voiced sorrow at the shooting, and the studio behind the movie said it was withholding box office data out of respect for the victims.

"Words cannot express the horror that I feel," British actor Christian Bale said in a statement. "I cannot begin to truly understand the pain and grief of the victims and their loved ones, but my heart goes out to them."

The horrific attack in Aurora has revived the debate over gun control in the United States, and drew condemnation by Obama and his Republican White House rival Mitt Romney. A further 58 people were wounded in the shooting.

Bomb experts spent Saturday inching their way into Holmes's apartment.

A small boom from a "controlled detonation" was heard by reporters outside the apartment block and pieces of debris were blown out of one of the windows through which police had been assessing the booby trap set up inside.

Aurora police chief Dan Oates did nothing to hide his anger at what the authorities found inside the apartment.

"Make no mistake, this apartment was designed to kill whoever entered it. And who was most likely to enter that location after he planned and executed this horrific crime? It was going to be a police officer," he said.

"And if you think we're angry, we sure as hell are angry, about what has happened to our city, what has happened to these wonderful people who live here, and also what he threatened to do to one of our officers."

The 12 fatalities from the midnight theater rampage included six-year-old Veronica Moser-Sullivan, according to the Arapahoe County Coroner's Office, which said they all died of gunshot wounds.

"She was beautiful and innocent," the girl's great aunt Annie Dalton told The Denver Post, recalling how the blonde child "loved to dress up and read, and was doing well at school."

Her mother Ashley, 25, was shot in the neck and abdomen and is in critical condition at Aurora Medical Center, drifting in and out of consciousness, and is unaware that her daughter is dead, according to the newspaper.

Many of those wounded in the mass shooting are expected to suffer long-term consequences.

Authorities said Holmes bought more than 6,000 rounds of ammunition online, as well as four guns in the two months before the rampage.

The shooter, dressed in black and wearing body armor and a gas mask, burst into the theater barely 20 minutes into the screening, throwing two tear gas-type devices before opening fire with several weapons.

Police arrested Holmes by his car at the rear of the theater. He offered no resistance.

New York City Police Commissioner Raymond Kelly said Holmes had painted his hair red and claimed he was the Joker, Batman's sworn enemy.

The suspected gunman "clearly looks like a deranged individual," he added.

Aurora police chief Oates said Holmes had received a large number of packages over the past four months, which could explain how he obtained the large amount of ammunition used in the attack.

"We also think it begins to explain some of the materials that he had in his apartment," he said. "What we're seeing here is evidence of, I think, some calculation and deliberation."

Oates flatly declined to speculate on any motive for Holmes, who is due to make his first court appearance on Monday.

Out of "an abundance of caution," bomb-sniffing dogs had made a sweep of buildings at the University of Colorado medical school, which Holmes attended until last month, but did not find anything unusual, Oates said.

Holmes had no criminal record aside from a citation for speeding in October 2011, according to police.

Cinemas in New York tightened security at Batman showings, and the AMC theater chain announced a ban on face masks and fake weapons, while a French premiere was canceled on Friday.

Aurora is 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.