DENVER (AP) — One of three teens who was charged with killing a 20-year-old woman while throwing large rocks at passing cars in Colorado pleaded guilty on Friday to reduced charges under a plea agreement, prosecutors said.

The deal reached with Zachary Kwak, 19, requires him to cooperate in the prosecution of the two other teens still being prosecuted on first-degree murder charges in the death of Alexa Bartell. That could involve being called to testify against Nicholas “Mitch” Karol-Chik or Joseph Koenig, a spokesperson for District Attorney Alexis King, Brionna Boatright, said.

Karol-Chik and Koenig have both pleaded not guilty.

Kwak pleaded guilty to first-degree assault in Bartell’s death on April 19, 2023, acknowledging that he acted in a way that created a grave risk of death, King’s office said. He also pleaded guilty to second-degree assault and attempted second-degree assault in the cases of rocks that prosecutors say were thrown from the trio’s truck earlier that night. Three other people were injured.