The key to understanding the death penalty trial of James Holmes, the man who killed 12 people and wounded 70 more at a packed Batman movie premiere, will be to learn what was going on inside his mind at the time.

Opening statements start Monday in the long-awaited trial of the 27-year-old, who has pleaded not guilty by reason of insanity. His July 20, 2012, attack in suburban Denver was among the deadliest in American history.

Holmes' lawyers say he was psychotic and could not tell right from wrong. But prosecutors say he carefully planned the shooting for months.

They will ask the final 12 jurors selected from a pool of 9,000 citizens to sentence Holmes to death. If they decide instead on insanity, he will be committed to a state mental hospital.