The first of some 9,000 potential jurors will report to court in Colorado as selection begins for the trial of James Holmes, accused of killing 12 people in July 2012 at an Aurora movie theater. (Reuters)

The trial of James Holmes, who is accused of killing 12 people and injuring scores more inside a Colorado movie theater, is finally getting underway on Tuesday.

A series of delays pushed the start of jury selection into this month, more than two and a half years after the shooting spree during a screening of “The Dark Knight Rises” in Aurora, Colo. The trial has been postponed multiple times, with some of these delays centering on Holmes’s mental state and the different psychological evaluations that have taken place.

Holmes has pleaded not guilty due to insanity, while prosecutors are seeking the death penalty. In a letter published last month, Holmes’s parents asked that their son not be executed, arguing that he should instead be institutionalized or imprisoned for the rest of his life.

Robert and Arlene Holmes wrote that the death penalty is “morally wrong, especially when the condemned is mentally ill.” In the letter, Holmes’s parents also criticized the decision to hold a long trial that would force people “to relive those horrible moments in time.”

Colorado has only carried out one execution since 1976.

This trial offers something rare: an alleged gunman, accused of carrying out a mass shooting, entering a courtroom to face a judge and jury. An FBI study of “active shooter” situations looked at 160 incidents between 2000 and 2013, a list that named the Aurora shooting as the deadliest during that period. More than half ended when the gunman stopped shooting, often because he committed suicide or fled; nearly half of the shooters looked at by this study ended their own lives.

Holmes has been charged with murder, attempted murder, using a deadly weapon for a violent crime and possessing explosive devices. Witnesses have said that the gunman calmly and silently walked through the theater as he fired.

Even as the trial is officially beginning this week, it will still take until the summer — nearly three years after the shooting — for the courtroom trial proceedings to truly begin. And the entire process could last through much of the year.

Jury selection is expected to take between four and five months, with the lengthy process stretching out because of the massive pool of potential jurors. District Judge Carlos A. Samour Jr. said in November that the court would summon 9,000 people, writing in an order that it would “be much easier to call off prospective jurors who are not needed than it will be to adjust if there are insufficient prospective jurors.”

Jurors will file into the courthouse for two or three hours at a time to hear instructions and fill out a questionnaire. They will be told not to talk to the media and not to read about the trial (including jury selection) in the news, according to Samour’s prepared remarks. Since the trial is expected to receive quite a bit of coverage, avoiding news about it “is likely to be a particularly daunting challenge that will require GREAT EFFORT on your part,” Samour wrote. (He repeated “GREAT EFFORT” two additional times in stressing the difficulty this could entail.)

In many ways, this trial parallels one happening on the other side of the country. A similarly large pool of potential jurors is being considered for the Boston Marathon bombing trial, which also involves a young man facing the death penalty and charged with attacking a crowded, public place.

Samour has said that the Colorado court expects to have 24 people — a dozen jurors and a dozen alternates — selected by June at the latest, when opening statements are expected. The trial is expected to last until September or October, he said.