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.