LITTLETON, Colo. (KDVR) — Jury selection is underway for one of two suspected school shooters who opened fire inside of STEM School Highlands Ranch in May of 2019, shooting nine students.

Alec McKinney, who was 17 years old at the time of the shootings, pleaded guilty in February to first-degree murder and was sentenced to life in prison with the possibility of parole for the shooting.

His alleged accomplice, Devon Erickson, is now standing trial. Erickson was 18 at the time of the shooting, and facing a possible life sentence if convicted.

Eighteen-year-old Kendrick Castillo died in the attack while attempting to shield classmates from the gunfire.

His parents, John and Maria Castillo, plan to attend every day of Erickson’s trial to make sure their son receives justice.

“We know he’s guilty. We know he’s a cold blooded killer,” said Maria Castillo.

Life has not been easy for the Castillo family in the two years since their son’s death, but it’s now about to get even more difficult because of the trial.

“It’s exhausting. We’re going to hear through this trial about mental health. They’ve already alluded to the idea it was an accident and that he didn’t mean to pull the trigger and all those crazy things we know that aren’t the truth,” said John Castillo.

The Castillo’s credit their strong Catholic faith with helping them get through the past two years.

“I think we’re closer than ever,” said Maria.

However, the one thing they both agree they struggle with is the idea of forgiving their son’s killers.

“Saying I’m going to forgive him, I’m lying,” said Maria.

“I know what our religion teaches us and it’s a very hard line. We’re not there yet, and I don’t know, maybe three, four, five years from now. It pits you against the thing you love and depend on the most and that’s our faith,” said John.

The Castillo’s say they are also frustrated with the court proceedings so far. Erickson’s trial has already been delayed several times because of the pandemic, and on Tuesday jury selection didn’t happen as planned because of a court mix-up.

John also says he’s frustrated Erickson is allowed inside the courtroom for jury selection, while he and his wife are forced to watch the proceedings on a monitor from another building.

“I feel cheated and robbed every time I see that. I feel like if I can see someone who’s been doctored up to look 12 and innocent, why can’t the jurors look the other way and see me and my wife who have experienced the ultimate loss,” he said.

The trial is expected to last about 5 weeks, with opening arguments likely beginning late this week or Monday next week.