All 24 charges carry a maximum death penalty sentence.
Prosecutors filed the "extreme indifference" charges in case Holmes' defense attorneys argue that their client was incompetent to form premeditation necessary to convict him of first-degree murder.
Holmes, 24, is accused of setting off two tear gas canisters in the Century 16 movie theater in Aurora on July 20 at a midnight premiere showing of the new Batman movie, "The Dark Knight Rises."
He then opened fire on the packed crowd, specifically targeting anyone who tried to flee out of the doors. Investigators believe more victims would have been shot but one of Holmes' weapons jammed.
When he was taken into custody in the parking lot outside the theater, Holmes told police that his apartment was booby-trapped. Bomb squad members spent three days removing explosive devices from the location.
Insanity Defense Planned?
Observers expect his lawyers to ask for a mental evaluation of Holmes, whom they said was seeing a psychiatrist at the University of Colorado, where he was a former neuroscience graduate student.
Victim family members present at Holmes' first court appearance expressed support for the legal system.
"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," the family of Gordon Cowden said in a statement.
12 Killed in Colorado Theater Massacre
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Photo: Mug Shot