Holmes' trial had been set to begin with jury selection on Feb. 3.
Holmes, 25, pleaded not guilty by reason of insanity to charges of killing 12 people and injuring 70 during a packed midnight showing of a Batman film at an Aurora theater in July 2012.
Prosecutors are seeking the death penalty, and they want Holmes to undergo further evaluation of his sanity.
The findings of Holmes' first mental health evaluation, conducted at a state hospital, have not been publicly disclosed. But the fact that prosecutors want further evaluation suggests the first exam might have found Holmes was insane.
Such a finding could make it much harder for prosecutors to persuade a jury to convict Holmes of murder and sentence him to death.
District Attorney George Brauchler said at Thursday's hearing that prosecutors are challenging one of three conclusions in the state evaluation. He did not elaborate. Both sides are barred from speaking about the case outside court.
District Judge Carlos Samour Jr. scheduled hearings on further testing and other pre-trial issues for Dec. 17 and 18.
Holmes' attorneys don't dispute that he committed the shootings, but his plea makes psychiatric evaluations - which assess whether Holmes was sane at the time of the shootings - the most important pieces of evidence.
If jurors were to find that Holmes was insane, he would be committed indefinitely to the state hospital. If doctors there ever concluded Holmes' sanity had been restored, he could one day be released, but that is considered unlikely.
Colorado law defines insanity as the inability to tell right from wrong because of a mental disease or defect. An evaluation by the state mental hospital is mandatory for anyone who pleads insanity. Holmes underwent his last summer.
Law enforcement officers have testified that Holmes planned the attack for months, stockpiling guns and ammunition. They also say he elaborately booby-trapped his apartment with bombs designed to explode and divert police and fire crews from the theater.
Prosecutors want to use that evidence to undermine the insanity claim by showing the attack was meticulously planned.
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