Jury selection has begun in the trial of James Holmes, who is accused of gunning down 12 people and wounding many more after opening fire in a crowded Colorado movie theater in 2012.

About 9,000 people were mailed summonses to serve on the jury, and a fraction of them were on hand to hear instructions from State District Judge Carlos A. Samour Jr., as well as fill out a questionnaire as part of a months-long jury selection process, according to a Wall Street Journal report.

Holmes, 27, is facing 166 counts of murder and attempted murder, as well as other charges. He is accused of shooting attendees of the film “The Dark Knight Rises” at a theater in Aurora, Colo.

Holmes was wearing a blazer and dress pants instead of the typical prison jumpsuit and started blankly as the judge discussed the ground rules, according to the report.

The 9,000-strong jury pool is one of the largest in U.S. history, which shows how much the mass shooting has impacted the state.

Several potential jurors were excused before Tuesday because they were witnesses for the prosecution, and the judge announced that the pool had been reduced to 7,000 due to some of these excusals and some undeliverable summonses.

About 70 people were injured the attack, which create a nationwide debate about gun control — an issue that has come up before in the state, which was the site of the Columbine High School massacre in 1999 that jump-started the gun control movement and led to the fiercely anti-gun documentary “Bowling for Columbine” by Michael Moore.

Judge Samour warned jurors not to talk about the case and not to read news reports on it, stating that even one violation of the rules could create major problems. It will be challenging to find jurors who can be impartial on the case due to its notoriety.

Holmes has pleaded not guilty by reason of insanity.

The trail should begin by June, Judge Samour stated.