James Holmes, Colo. movie theater shooting suspect, to be formally charged today

(CBS/AP) DENVER - Colorado prosecutors are expected to file formal charges Monday against James Holmes, the 24-year-old former neuroscience student accused of killing 12 people and injuring 58 at an Aurora movie theater. Holmes is not expected to enter pleas during his second court appearance.

Pictures: 12 dead, dozens wounded in shooting at Batman movie

Attorneys will also argue at this morning's hearing over a defense motion to find out who leaked information to the media about a package Holmes allegedly sent to his psychiatrist at the University of Colorado, Denver.

Authorities seized the package on July 23, three days after the shooting, after finding it in the mail room of the medical campus where Holmes studied. Several media outlets reported that the package contained a notebook with descriptions of an attack, but Arapahoe County District Attorney Carol Chambers said in court papers that the parcel hadn't been opened by the time the "inaccurate" news reports appeared, The Associated Press reported.

District Court Judge William Blair Sylvester 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.

Legal analysts expect the case to be dominated by arguments over Holmes' sanity. 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, according to the AP.

Holmes allegedly began stockpiling gear for his assault four months ago, and authorities say he bought his weapons in May and June, well before the midnight shooting spree during a showing of the new Batman film. He was arrested by police outside the theater.

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."

Experts said there are two levels of insanity defenses. Holmes' public defenders could argue he is not mentally competent to stand trial. If they 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.

On Friday, court papers revealed that Holmes was seeing a psychiatrist at the university. But they did not say how long he was seeing Dr. Lynne Fenton and if it was for a mental illness.

Complete coverage of James Holmes on Crimesider