Mitchell Kraus didn't know he'd been shot until he'd already run a couple of blocks from STEM School Highlands Ranch on May 7, 2019.

When he realized something was wrong, he took stock of his body, moving his hand to his back. It came back red with blood.

The next thing he remembers wanting to do was call his mom.

“She picks up the phone and she answers in her cheery voice like everything's normal,” he said through tears on May 28. “I opened my mouth and the only thing I could manage to say was 'mom help.'”

Kraus told the story of the shooting — sometimes through deep heaves and sobs — to a jury as a part of the first day of testimony in the trial against Devon Erickson. Erickson faces dozens of charges related to the school shooting from two years ago, which left eight students injured and one, Kendrick Castillo, dead.

Alec McKinney, Erickson's codefendant, pleaded guilty last year to charges related to his participation in the shooting, including first-degree murder. He was sentenced to life in prison with a chance of parole after 40 years.

The rest of the day consisted of testimony from others like Kraus. The prosecution called on several staff members and students of the school to walk the jury through the day of the shooting and how it unfolded from their perspective.

An IT director who tried to save a student's life with his bare hands. A school nurse who tended to Erickson only minutes before he pulled a gun on a class of students. A student who said he had to pry a handgun from Erickson's hands before he would surrender.

In the first day of testimony, as in the previous day's opening statements, the attorneys representing Erickson didn't challenge whether or not their client took part in the shooting. Instead, they've called attention to his intent and pointed to evidence that they say proves he was pressured by McKinney to participate. They've also claimed that he only took part due to factors including his age, a lack of sleep and persistent drug use.

MORE: Full coverage of the STEM School shooting and its aftermath

Leadup to shooting

Karen Lewis, the school nurse at STEM, was called into the high school's office in the afternoon of May 7 after someone reported a sick student, she told the jury during the first day of testimony.

The student, Erickson, reported feeling hot and exhausted. He was also visibly shaking and had dark circles under his eyes, according to another staff member who testified.

Minutes before, another student, Kraus, had been assigned to walk Erickson to the nurse's office to make sure he got there OK.

When the nurse arrived, Erickson told her about troubles he was having at home and told her he just needed to rest for a few minutes.

“My heart felt for him,” she said.

After a few minutes, Erickson reported to the nurse that he was feeling better and would return to class. When asked by the prosecution if she noticed anything odd about Erickson, Lewis said he just looked exhausted and upset about his situation at home.

“I'm an experienced nurse and I didn't see anything,” she said. “And I would have.”

A few minutes later, Lewis heard reports that someone was shooting people in the school.

Room 107

Like the other students in British Literature class that day, Brendan Bialy was watching “The Princess Bride” when Erickson re-entered room 107 and pulled a gun out, he told the jury.

“He said `nobody f------ move,'” Bialy told the jurors.

Testimonies vary on whether or not Erickson was pointing the gun directly at classmates or above their heads.

In an instant, Kendrick Castillo had jumped to his feet and was rushing toward Erickson. Bialy and another student, Josh Jones, followed. In the tussle, the gun went off several times and Castillo was hit in the chest, Bialy said.

Bialy described fighting with Erickson, punching him repeatedly and eventually prying the handgun from his fingers while the defendant continued to fight.

“He was deliberately turning the firearm back toward Josh and I's head with his finger attempting to reach the trigger,” Bialy said of Erickson.

Once Erickson's gun had been taken away, he started saying he had been forced to participate and that there was another shooter, Bialy told the jurors.

Before long, Bialy said, he was left in the classroom with only Erickson, Jones and Castillo as everyone else fled. Eventually, Mike Pritchard, the school's IT director, came into the room trying to find what had happened.

Pritchard, who also testified that day, immediately attended to Castillo when he entered the room, he said.

Though he could see that Josh Jones had also been shot, Castillo's injury was clearly more urgent. He asked Bialy to find something to put pressure on the wound.

“He wasn't able to find anything so at that point I used my hands to start applying pressure,” Pritchard said.

As Pritchard kneeled by Kendrick, the young man was still somewhat responsive.

“It was like he was trying to say something but it wasn't anything I could actually understand,” he told the jurors.

Eventually, though, the young man stopped breathing. Pritchard tried to find a pulse but couldn't.

Testimonies in the trial were expected to restart the morning of June 1, the day after the long Memorial Day weekend.