Officials said Holmes, 24, put on body armor and killed 12 people, injuring 58, after opening fire in a movie theater July 20. The theater was crowded for the midnight premiere of "The Dark Knight Rises."
The suspect already faces multiple murder and attempted-murder charges.
Holmes, a former neuroscience graduate student, sat quietly during the 75-minute court hearing, not even reacting when defense attorney Dan King said "we cannot begin to examine the nature and depth of Mr. Holmes' mental illness" because of the preliminary investigation yet to do, USA Today reported.
The newspaper said defense attorneys and prosecutors objected to media efforts to learn more details about the case. King said, "Many of the witnesses are in hiding because of the conduct of the media in this case."
The judge set a court hearing for Oct. 25 to determine whether police improperly leaked information to the media, and set a preliminary hearing for Jan. 7.