DENVER — Launching a case that legal analysts expect to be dominated by arguments over the defendant’s sanity, Colorado prosecutors plan to file formal charges Monday against James Eagan Holmes, the former neuroscience student accused of killing 12 people and wounding 58 others at an Aurora movie theater.

Attorneys will also argue over a defense motion to find out who leaked information to the news media about a package the 24-year-old former neuroscience graduate student allegedly sent to his psychiatrist at the University of Colorado Denver.

Authorities seized the package July 23, three days after the shooting, after finding it in the mailroom of the medical campus where Holmes studied. Several media outlets reported that it 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.

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 Batman film “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., 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.”