Some 9,000 people have received juror summons as jury selection began in the Colorado cinema massacre trial.

District Judge Carlos Samour, who is handling the massacre case, said 150 jurors will be selected from thousands of potential jurors, which will then be whittled down to 24. Only 12 jurors will be sitting in the actual trial, and 12 will serve as alternates.

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The selection could be painstaking, as it is expected to last until May or June.

Twelve people were killed and at least 70 others were injured in the 2012 bloody rampage at a movie theater in Aurora, Colorado. James Holmes, a former neuroscience student, admitted he was the lone shooter, but pleaded not guilty by reason of insanity. 

In a statement, Samour says he expects "passionate and zealous" arguments from the prosecutors and defense team. Samour also reminded prospective jurors, justice must be served.

"That's what this whole process is about: Justice," Samour said.

The first batch of potential jurors walked into the courtroom of the Arapahoe County Justice Center. If chosen, they will decide on Holmes' fate. Should they find Holmes guilty of the crime, they will also decide if he should be executed.

Holmes was also present in the courtroom. He was no longer wearing his orange-dyed hair, but he remained expressionless as he sat upright in a dark jacket and khaki slacks, matched with trimmed hair and red-framed glasses.

Last month, Holmes' parents pleaded for their son's life and appealed to prosecutors to drop the death penalty. They were calling on the prosecutors to offer Holmes a plea deal instead, that will allow him to seek treatment while locked up.

"We do not know how many victims of the theater shooting would like to see our son killed. But we are aware of people's sentiments," they wrote. "He is not a monster. He is a human being gripped by a severe mental illness."

Tagscolorado shooting, Colorado shooting trial, James holmes, jury selection