Opening arguments were due to begin Monday in the James Holmes murder trial, almost three years after authorities say the Colorado man opened fire at a movie theater's midnight showing of "The Dark Knight Rises" in July 2012. Twelve jurors will decide the fate of Holmes, who has pleaded not guilty by reason of insanity. The trial is expected to draw a lot of media attention and could last as long as four months, USA Today reported.

Here are five basic things to know about the trial.

1. The defendant: At the time of the shooting, Holmes was a 24-year-old who had recently dropped out of his neuroscience graduate program at the University of Colorado, where he also worked. He'd received multiple deliveries to home and work that police said allowed him to accumulate 6,000 rounds of ammunition and four weapons, CNN reported. Holmes had no social media accounts except for suspected profiles on and Adult Friend Finder. 

CBS News reported that before the shooting, he'd seen at least three school psychiatrists. One, Lynne Fenton, said she alerted police that Holmes was homicidal and could be dangerous.

2. The crime: On July 20, 2012, police said, Holmes entered through an emergency exit at the Century 16 cinema in Aurora, Colorado, wearing a gas mask and several pieces of protective gear. He set off smoke bombs and and began shooting at viewers, killing 12 and injuring 70, ABC News reported. At first, some people thought it was part of a movie promotion.

Police arrived within 90 seconds of the first 911 call and arrested Holmes near his car outside the theater. Holmes' hair was dyed orange like the Batman villain the Joker, and his apartment was later found to be booby-trapped, police said. Prosecutors alleged that he'd planned the theater attack for months.

3. The charges: Holmes was charged with more than 140 counts of murder and attempted murder -- two counts for each victim, WJLA-TV reported. He pleaded not guilty by reason of insanity in 2013.

4. The defense: Holmes' lawyers are not expected to contest the fact that he was the Aurora shooter. But they've said he was severely mentally ill and "in the throes of a psychotic episode" at the time, the Associated Press reported. Holmes has attempted suicide while in jail and has undergone two psychiatric evaluations in connection with the case. He was found competent to stand trial.

5. The sentence: Prosecutors are seeking the death penalty, which would be one of two options for jurors should they find Holmes guilty. The AP reported that if jurors convict Holmes, they can sentence him to execution or life in prison. If he's found not guilty, he'll go to a mental hospital indefinitely.