AURORA, Colorado (AFP) - US President Barack Obama will visit Colorado on Sunday to meet victims of the Batman cinema massacre, a day after police entered the gunman's booby-trapped apartment.

The visit also comes after police revealed the names of the 12 people who died -- including a six-year-old girl whose mother was seriously injured -- while asserting that the alleged killer had been planning the attack for months.

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 early Friday 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.

Bomb experts spent Saturday inching their way into Holmes's apartment. A small boom from a controlled detonation blew out a few pieces of debris, and a short time later police announced they had defused all major threats.

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.

Early Sunday police dismissed media reports that another person, a fellow medical PhD student, had helped Holmes in his killing spree, and had called the police and threatened more violence if Holmes was not released.

"There are multiple unconfirmed and inaccurate news reports about a second suspect," a spokeswoman said, adding: "An associate of Holmes was interviewed this evening. There is no reason to believe that he is involved."

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.

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.

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.

And he left few obvious clues to any motive or to his general state of mind. He appears to have had no Facebook, Twitter or other social media accounts in his name. High school and university friends have unanimously expressed shock and bewilderment at his arrest, saying he did not resemble the person that they knew.

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 and the Philippines has ordered tighter security at all shopping malls.

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.