The 24-year-old neuroscience student allegedly burst into a midnight viewing of the latest Batman film, The Dark Knight Rises, then opened fire into the packed auditorium with a high-powered assault rifle.
Ten of the 58 who were wounded in the July 20 attack remain hospitalised and four are in a critical condition, leaving open the possibility that the huge charge sheet could yet grow longer.
Among those clinging to life is Ashley Moser (25) whose six-year-old daughter Veronica was killed in the massacre. Moser was paralysed after she was shot in the neck and stomach and has since lost her unborn child.
Judy Woodard, a Moser family friend said she was grateful to be able to see the accused face justice.
"So many times when something like this happens, the person ends up killing themselves, like Columbine," she said.
Don Lader (27) escaped the massacre unharmed but knew one of the victims, Navy cryptologist John Larimer, who died after using his body to shield his girlfriend.
"We get to be on the other side now, on the attack," said Lader, who wore a Batman t-shirt to court in defiance branding Holmes "a coward".
The family of Gordon Cowden, the oldest victim at 51, issued a statement in which they expressed confidence that justice would prevail.
"We feel confident that our judicial system, as will God and the public, [will] see to it that this evil man receives the punishment he deserves and our hope and prayers are that one day, he will face God with remorse and tremendous sorrow for his actions," they said.
Authorities say Holmes claimed he was "The Joker", Batman's sworn enemy in the comic book series that inspired director Christopher Nolan's film trilogy.
He stunned observers at his bizarre first court appearance last week, when he alternated between staring out wild-eyed and slumping drowsily under a mop of brightly-dyed orange and red hair.
Holmes appeared more focused on Monday, but he spoke little, saying "yes" just once after Judge William Sylvester asked if he was willing to waive his right to a preliminary hearing.
His hair, though still reddish-orange, was combed forward and less wild than a week ago. His expression shifted from wide-eyed and expectant to calmly staring ahead.
Prosecutors have said it will be several weeks before a decision is made on whether or not to seek the death penalty for Holmes. Only one person has been executed in Colorado since capital punishment was reinstated in 1976.
But the 40-page charge sheet underlined their efforts to ensure he is severely punished.
Holmes faces two counts of first degree murder for each of the 12 people killed – one each for their deliberate killing and one more for killing through "depraved indifference" to the lives of his victims.
'Crime of violence'
He also faces 116 attempted murder charges for wounding 58 more people in the theatre with indiscriminate firing, one count of possession of an explosive device and one sentence enhancement count for a "crime of violence".
There has been speculation that stress over failing an important oral exam may have been the trigger that caused Holmes, a promising neuroscience student who had won a prestigious government grant, to become unhinged.
ABC News' affiliate in Denver, KMGH-TV, reported that Holmes purchased an automatic rifle on June 7, hours after failing the key oral test. Three days later, he dropped out of his university program.
Prosecutors have been battling defence lawyers over a package Holmes sent to his psychiatrist at the University of Colorado portending the midnight massacre at a cinema in Aurora, outside Denver.
Attorneys for Holmes disclosed on Friday that he had been a patient of University of Colorado psychiatrist Lynne Fenton as they sought to gain access to a package he had mailed to her prior to the massacre.
Holmes's lawyers are accusing prosecutors of leaking to the media the existence of the package – reportedly containing macabre plans, including drawings of a stick-figure gunman mowing down victims.
Holmes is said to have stocked up on more than 6 000 rounds of ammunition over the Internet and to have bought four weapons in local gun shops.
Although the shooting has triggered some soul searching in the United States there has been no concerted political will to address the toxic gun law issue, especially four months out from a presidential election. – AFP