One of three high school seniors charged with throwing rocks at cars in Colorado last year — one of the incidents killed a 20-year-old woman — pleaded guilty on Friday to assault and attempt to commit assault.

All other charges against 19-year-old Zachary Kwak were dismissed as part of the plea deal, which will see him serve no less than 20 years in prison and no more than 32 years when he is sentenced in September, according to the District Attorney’s Office of Gilpin and Jefferson Counties.

Kwak allegedly threw the rock that killed Alexa Bartell on April 19, 2023.

Kwak and two other teens were initially charged with first degree murder and 13 other charges in a series of rock-throwing incidents, as CrimeOnline reported. Bartell was driving northbound on Indiana Street near the Rocky Flats National Wildlife Refuge, on the phone with a friend, she was struck in the head by a rock, killing her.

Her friend tracked the phone and found Bartell dead in her vehicle, which had veered into a field.

Investigators later learned the boys intentionally drove toward Bartell’s vehicle and slowed down to take a photograph of the crashed car as a “memento” of what happened. They did not stop to help.

That same day, two other victims were injured in earlier assaults involving rocks, while seven vehicles in total sustained damage from what authorities have characterized as “large landscaping rocks.” According to reports, the suspects may have been traveling in a moving vehicle at the time the rocks were thrown.

The boys were ultimately arrested after police identified them based on cellphone data and information from a friend who had been socializing with them before they began hurling rocks at vehicles.

According to KDVR, members of Bartell’s family were in the courtroom on Friday to hear Kwak’s guilty plea. He appeared in a jail uniform, handcuffed, telling the judge he understood his plea and what it meant. He won’t be sentenced until verdicts are rendered against his co-defendants, Joseph Koenig and Nicholas Karol-Chik.

Karol-Chik’s trial is scheduled to begin on June 7, and Koenig’s on July 19, although he has a motions hearing scheduled on July 3. All three of the boys were 18 at the time they killed Bartell.

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[Featured image: Alexa Bartell/Jefferson County Sheriff’s Office]