CASTLE ROCK, Colo. (KDVR) — On Friday, jurors viewed for the first time a showdown between a security guard and student during the shooting that took place at STEM School Highlands Ranch. 

That security guard described a tense conversation between him and the teenagers after they opened fire at the school.

The guard was one of the first to respond just seconds after the shooting began at the school two years ago.

His testimony came in the trial of the student accused of murdering a fellow classmate, Devon Erickson. 

Erickson is charged with murdering Kendrick Castillo, who tried to stop him. 

The dramatic video shows the moments the security guard at the high school approached the student with a gun. 

You can see that guard, Shanson Sundara, head toward the armed teen who follows orders to lie on the floor.

The guard then kicks a gun away. 

All the while, Sundara said the other student now convicted in the shooting, Alec McKinney, kept making threats. 

McKinney was one of two shooters.

“Alec was telling me we have more guns than you. We are going to kill you. You need to kill me,” Sundara said. 

Sundara would go on to fire two shots at someone who he mistook for another gunman. 

It was an officer, who was not hurt. But two students were. 

Teacher Gabrielle Leddy says she heard an argument — then terror and chaos followed. 

“I was walking and all of sudden I heard a loud noise,” Leddy said. “A loud boom. I did not recognize at first what it was. And I was sprayed with drywall on my face with powder. I looked in front of me and there was a bullet hole.”

A student in the classroom where the shooting took place described how Erickson held a gun moments before opening fire. 

“By the time he was fully standing up, he was fully facing me,” the student testified. “What was happening with the gun at that point? In the simplest of terms, he was holding it gangster style and kind of swung and then pointed, which was directed toward all of the students.” 

Today the teacher who was in the classroom where the shooting happened fought back tears describing how Erickson looked that day.

Lauren Harper also said she put McKinney in a head lock to try to stop the shooting. 

Seconds earlier she heard Erickson yell, “Nobody [expletive] move!”

That teacher says at one point she played dead to try and survive the shooting. 

One student asked her, “Are you dead?”

She told the juror one of her main goals was to get the school locked down.