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 Robert BrownFriday 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.

In exchange for Kwak’s guilty pleas to those charges, prosecutors dropped other more serious charges against him, including first-degree murder. Kwak faces between 20 and 32 years in prison, prosecutors said. He will not be sentenced until Sept. 3, after Karol-Chik and Koenig are scheduled to be tried separately.

Karol-Chik’s lawyer, Holly Gummerson, declined to comment on the plea deal or the allegations against her client. Lawyers representing Koenig did not immediately return a telephone call or emails requesting comment.

Prosecutors allege that all three drove around in Karol-Chik’s pickup truck the night Bartell was killed after loading it up with landscaping rocks they took from a Walmart.

The men were arrested several days after Bartell was hit while driving northwest of Denver and talking on the phone with a friend. After the call went silent, the friend tracked Bartell’s location with a phone app and found the suburban Denver woman dead in her car, which had crashed into a field.

Investigators have said Bartell was killed by the rock and not the crash.