US President Barack Obama landed in Colorado on Sunday to meet relatives of those gunned down in the Aurora cinema massacre, after residents flocked to church services to remember the 12 dead.
Another 58 people were injured when a gunman clad in body armor and a gas mask burst into the packed premiere of the new Batman film "The Dark Knight Rises" in the Denver suburb, and opened fire shortly after midnight on Friday.
The alleged gunman, 24-year-old James Holmes, emerged from a fire exit, threw two canisters of noxious gas into the crowd, fired one round in the air and then began shooting at random.
Obama's first stop was the University of Colorado Hospital in Aurora, where he met relatives of the victims, and where some of the survivors remain in critical condition.
Colorado governor John Hickenlooper, who joined the president, said earlier it was unlikely that Obama would stay for a community vigil on Sunday. But Obama was expected to make a statement to the media at the hospital.
Police revealed on Sunday that they had found Holmes's computer inside his booby-trapped apartment, which could provide crucial details about how he planned and executed the attack, reportedly over a number of months.
Holmes, who is due to make his first court appearance on Monday, is being held in solitary confinement for his own protection, a police spokeswoman said.
The town's churches were packed Sunday, with some worshippers sobbing -- many churchgoers said they knew someone involved in the shooting.
"My niece was in the theater but she was lucky to get out," said 15-year-old student Keshala Ferguson, among a capacity 500-strong congregation at the Queen of Peace Catholic church.
"It was nerve-wracking for her mom, but she's OK. The mass was amazing. Everybody needs to pray."
Calls for another look at America's gun laws are mounting in the aftermath of the tragedy as it emerged that the suspect bought his four weapons legally, as well as thousands of rounds of ammunition on the Internet.
Police on Saturday entered his apartment and disclosed that the fatalities included a six-year-old girl whose mother was among those critically injured.
Aurora is located 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.
City police chief Dan Oates on Sunday praised his men for spotting Holmes as a suspect in the "chaos" that followed the shooting, as officers could have mistaken him for an arriving SWAT officer, because of his full body armor.
New York Mayor Michael Bloomberg -- one of the few high-ranking US politicians that openly favors gun control -- demanded Sunday that both Obama and his Republican presidential rival, Mitt Romney, take action on the issue.
"This really is an enormous problem for the country, and it's up to these two presidential candidates," Bloomberg said on the CBS show "Face the Nation."
"They've said things before that they're in favor of banning things like assault weapons.
"Where are they now and why don't they stand up?" asked Bloomberg. "If they want our votes, they'd better."
Meanwhile actress Anne Hathaway became the latest cast member to voice her shock at the killings, after Warner Brothers, the studio behind the new Batman movie, said it was withholding box office data out of respect for the victims.
"My heart aches and breaks for the lives taken and altered by this unfathomably senseless act. I am at a loss for words how to express my sorrow," said Hathaway, who plays Catwoman.
Bomb experts cleared Holmes's apartment of all major explosives threats on Saturday, when a small boom from a controlled detonation blew out some debris.
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, threatening more violence if Holmes was not released.
The Arapahoe County Coroner's Office said all 12 fatalities, including six-year-old Veronica Moser-Sullivan, died of gunshot wounds.
Authorities said Holmes bought more than 6,000 rounds of ammunition online, as well as four guns in the two months before the rampage.
Police arrested Holmes by his car at the rear of the theater after the shooting. 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.