The man accused of gunning down 12 filmgoers and injuring dozens more at a screening of the latest Batman movie in Colorado was expected to make his first court appearance on Monday.
James Holmes, 24, is accused of committing the massacre at a packed midnight screening of "The Dark Knight Rises" on Friday in the town of Aurora, just outside Denver.
Witnesses have said that a man clad in full body armour emerged from a fire exit after the show began, threw two canisters of noxious gas into the crowd, fired one round in the air and then began shooting people at random.
A six year-old girl was among the victims of the attack, which left a further 58 people wounded.
Holmes has been held in solitary confinement since his arrest.
According to a report in the Washington Post, the gunman might have killed more people had his AR-15 assault rifle not jammed.
The rifle can fire 50 to 60 rounds per minute and is designed to hold large ammunition magazines.
Police found a 100-round magazine on the cinema floor, said the paper, citing paper Aurora police chief Dan Oates.
He made no mention of whether the gun had jammed.
Calls for another look at America's gun laws are mounting in the aftermath of the tragedy as it emerged that Holmes bought his four weapons legally, as well as thousands of rounds of ammunition on the Internet.
Over eight weeks he stocked up over the Internet on 6,300 rounds of ammunition: 3,000 for his .233 semi-automatic AR-15 rifle, another 3,000 for his two .22 Glocks, and 300 cartridges for his pump-action shotgun.
Police said on Sunday they had found Holmes's computer inside his booby-trapped apartment -- designed to kill anyone who entered -- which could provide crucial details about how he planned and executed the attack.
On Sunday, President Barack Obama visited Aurora, which is located just 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.
That shooting prompted consternation in the United States and the rest of the world, but ultimately had no effect on America's gun laws which are backed by a constitutional amendment defending the right to bear arms.
During his visit to Aurora, Obama met with survivors of the shooting, telling the nation that he hugged and "shed tears" with them and relatives of the victims.
But Obama and Republican presidential hopeful Mitt Romney faced pressure on Sunday from New York Mayor Michael Bloomberg -- one of the few high-ranking US politicians that openly favors gun control -- to take action.
"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."