DENVER >> At first glance, the Colorado movie theater shooting case seems simple. James Holmes admitted to the attack that left 12 dead and 70 injured.

It will be far more complicated for jurors, who will wrestle with whether he was insane when he barged into a packed movie theater, clad in combat gear, and opened fire on moviegoers in July 2012.

The answer to that will help give the most complete look yet of the man accused of one of the worst mass shootings in U.S. history. Experts say it is rare to have a mass shooter appear in court to face charges — many either are killed by police or commit suicide.

“The public is going to get an insight into the mind of a killer who says he doesn’t know right from wrong,” said Alan Tuerkheimer, a Chicago-based jury consultant. “It is really rare. It just doesn’t usually come to this.”

The first step begins on Tuesday, when 9,000 prospective jurors — what experts say is the largest jury pool in U.S. history — begin arriving at the courthouse in Centennial, in suburban Denver.

Whittling those numbers to the 12 jurors and 12 alternates is expected to take months. The trial could last until October.

In the 2-1/2 years since the shooting, the case has sparked an emotionally charged debate, with his parents begging for a plea deal that would save his life while many survivors and family members of victims have demanded that he be put to death.

Holmes, 27, was arrested as he stripped off his combat gear in the parking lot of the Century 16 movie theater in Aurora after he opened fire at the midnight showing of a new Batman movie.

He later pleaded not guilty by reason of insanity to multiple counts of first-degree murder and attempted murder.

If jurors find him guilty, they must then decide whether to recommend the death penalty. If Holmes is found not guilty, he would be committed indefinitely to the state mental hospital.

Under Colorado law, defendants are not legally liable for their acts if their minds are so “diseased” that they cannot distinguish right from wrong. Part of the reason the case has dragged on so long is the battle over whether that standard applies to Holmes.

Few details on those arguments have been made public. Prosecutors and defense attorneys remain under a long-running gag order, and court documents detailing the issue have stayed under seal.

Holmes’ sanity was evaluated by a state psychiatrist but the results were not made public. Prosecutors objected to the findings and persuaded a judge to order a second evaluation. Those results were contested by the defense.

Prosecutors previously rejected at least one proposed plea deal made by attorneys for Holmes, criticizing the lawyers for publicizing the offer and calling it a ploy meant to draw the public and the judge into what should be private plea negotiations.

Survivors of the attack and family members of victims have had a long time to get ready for a trial.

“We’ve all been to therapists and have talked to our families and have our support groups, so we’re prepared,” said Marcus Weaver, who was shot in the arm and whose friend, Rebecca Wingo, died in the attack. “It’s gonna be quite the journey.”

It could take until June to find the jurors and alternates who were not biased by the widespread news coverage of the shooting. Equally challenging will be finding jurors who were not personally affected by the attack.

Judge Carlos Samour called nearly nine times as many prospective jurors as were summoned in the ongoing Boston marathon bombing trial. That meant the county’s 600,000 residents had a nearly one-in-50 chance of being selected.

Among those summoned were 13 people who were either witnesses to the attack or have family members who work in the prosecutor’s office. They were quickly excused.

Potential jurors won’t be allowed to read or talk about the case until they know they’ve been dismissed. That will be difficult in the era of social media, since they have no control over what pops up on their Facebook or Twitter feeds. The final panel won’t be sequestered.

During the selection process, Holmes’ attorneys will focus on picking jurors who are morally opposed to capital punishment, even as prosecutors fight to ensure those on the panel are “death-penalty eligible,” meaning they would be open to executing Holmes.

Enduring the trial from the jury box would seem even more difficult than the selection process.

“This is going to be a life-changing event for every juror who sits on the trial,” said Joseph Rice, managing partner of the Jury Research Institute, a California-based trial consulting firm. “They are going to deal with life-and-death issues that will forever be part of their experience.”