(CNN) — One of three men charged in the killing of a woman on a Colorado highway last year after allegedly throwing rocks at cars, has pleaded guilty, prosecutors say.

Nicholas Karol-Chik, Joseph Koenig and Zachary Kwak each originally faced more than a dozen charges, including murder, in the death of Alexa Bartell, 20, who was found dead in her car just northwest of Denver.

They were also charged with injuring three others as multiple moving vehicles were struck by rocks on April 19, 2023.

The three were 18 at the time of their arrests.

Kwak, now 19, pleaded guilty to first-degree assault, second-degree assault, and criminal attempt to commit assault in the second degree on Friday, according to a news release from the Colorado First Judicial District Attorney’s Office.

“As part of today’s plea, Kwak agreed that with regard to the death of Bartell, the defendant acted knowingly, under circumstances manifesting extreme indifference to the value of human life, by engaging in conduct which created a grave risk of death,” the district attorney’s office said.

Authorities said Kwak, who allegedly threw the rock that killed Bartell, said, “We have to go back and see that” and took a photo of the crash. He told investigators he thought the other two men “would want it as a memento,” according to an affidavit.

Kwak faces 20 to 32 years in prison, the release said.

He is scheduled to be sentenced on September 3 after the separate trials of Karol-Chik and Koenig, the release said.

CNN’s Andy Rose contributed to this report.

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