If the doctors do determine that Holmes was insane, a jury could still find him guilty.
The insanity plea was widely expected given the compelling evidence against Holmes. He is charged with multiple counts of murder and attempted murder. Prosecutors are seeking the death penalty.
Holmes's attorneys have said in court hearings and written in court documents that Holmes is mentally ill. He was being seen by a psychiatrist before the attack during a midnight screening of the latest Batman movie that killed 12 people and injured 70.
Prosecutors say Holmes spent months buying guns and thousands of rounds of ammunition, donned police-style body armor and opened fire in a crowded Aurora theater during the screening.
Holmes faces more than 160 counts of murder and attempted murder and could be executed if he is convicted.