Investigators said Holmes began stockpiling gear for his assault four months ago and bought his weapons in May and June, well before the shooting spree just after midnight during a showing of "The Dark Knight Rises." He was arrested by police outside the theater.
Analysts said that means it's likely there's only one main point of legal dispute between prosecutors and the defense.
"I don't think it's too hard to predict the path of this proceeding," said Craig Silverman, a former chief deputy district attorney in Denver. "This is not a whodunit. ... The only possible defense is insanity."
Under Colorado law, defendants are not legally liable for their acts if their minds are so "diseased" that they cannot distinguish between right and wrong. However, the law warns that "care should be taken not to confuse such mental disease or defect with moral obliquity, mental depravity, or passion growing out of anger, revenge, hatred, or other motives, and kindred evil conditions."
Experts said there are two levels of insanity defenses.
Holmes' public defenders could argue he is not mentally competent to stand trial, which is the argument by lawyers for Jared Loughner, who is accused of killing six people in 2011 in Tucson, Ariz., and wounding several others, including Rep. Gabrielle Giffords. Loughner, who has pleaded not guilty to 49 charges, has been diagnosed with schizophrenia and is undergoing treatment at a Missouri prison facility in a bid to make him mentally fit to stand trial.
If Holmes' attorneys cannot convince the court that he is mentally incompetent, and he is convicted, they can try to stave off a possible death penalty by arguing he is mentally ill. Prosecutors will decide whether to seek the death penalty in the coming weeks.
Holmes was not expected to enter pleas on Monday.
He ultimately could verbally enter a plea to the anticipated dozen first-degree murder charges, or his attorneys could enter it for him. Prosecutors may file multiple counts of attempted first-degree murder and other charges against Holmes, who booby trapped his apartment with the intent to kill any officers responding there the night of the theater attack, Aurora police said.
Sam Kamin, a law professor at the University of Denver, said there is "pronounced" evidence that the attack was premeditated, which would seem to make an insanity defense difficult. "But," he said, "the things that we don't know are what this case is going to hinge on, and that's his mental state."
Friends in Southern California, where Holmes grew up, describe him as a smart, sometimes awkward youth fascinated by science. He came to Colorado's competitive neuroscience doctoral program in June 2011. A year later, he dropped out shortly after taking his year-end exam.
Sylvester has tried to tightly control the flow of information about Holmes, placing a gag order on lawyers and law enforcement, sealing the court file and barring the university from releasing public records relating to Holmes' year there.
A consortium of media organizations, including The Associated Press, is challenging Sylvester's sealing of the court file.
On Friday, court papers revealed that Holmes was seeing a psychiatrist at the university. But they did not say how long he was seeing Fenton and if it was for a mental illness or another problem.
The University of Colorado's website identified Fenton as the medical director of the school's Student Mental Health Services.
An online resume listed schizophrenia as one of her research interests and stated that she sees 10 to 15 graduate students a week for medication and psychotherapy, as well as five to 10 patients in her general practice as a psychiatrist.
Online: Complaint against Holmes: http://bit.ly/OueB2J
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