GOLDEN>> Katharine Decker set a bagged rock about the size of a cantaloupe in front of the jury with a heavy thump Monday.

This rock, Jefferson County’s chief deputy district attorney told the jurors, is the rock that killed 20-year-old Alexa Bartell. The defendant, Joseph Koenig, threw it through her windshield two years ago and he is guilty of murder, Decker said during opening statements in Koenig’s trial.

“This rock blew through her head like a cannon,” she said, noting the fatal attack was one of several in an ongoing spree that night.

Koenig is guilty, his attorneys conceded in their own opening statements — but not of first-degree murder. The then-18-year-old didn’t intend to hurt anyone that night, defense attorney Thomas Ward said. He and two other teenagers didn’t think at all about the potential danger of hurling rocks at other drivers. They thought only about hitting cars, not people.

“All three of the boys in that truck are guilty of causing Alexa Bartell’s death,” Ward told jurors. “We are not running from that. But the evidence will show Alexa Bartell’s death, as tragic as it was, was not first-degree murder.”

Koenig’s jury trial started Monday in Jefferson County District Court, where the 20-year-old is charged with first-degree murder, nine counts of attempted murder, three counts of assault, six counts of attempted assault. He faces life in prison if he is convicted of first-degree murder.

Ward suggested jurors should find Koenig guilty only of the lesser offense of manslaughter. He said Koenig’s mental state that night didn’t match what is legally required for a first-degree murder conviction.

Koenig is accused of carrying out a series of attacks in which he and two other teenagers threw rocks and objects at passing cars, including one attack on April 19, 2023, that killed Bartell. Koenig and the other teenagers threw at least 10 objects at cars on at least three different nights between February and April 2023, prosecutors allege. Decker showed photos of cars damaged in the spree. Some had rock-sized holes in their windshield, or smashed bumpers or crushed side mirrors.

Prosecutors say Koenig threw the rock through Bartell’s windshield as the two vehicles passed each other around 10:45 p.m. on Indiana Street near the Rocky Flats National Wildlife Refuge. Koenig’s defense team alleges it was another teenager in the car who threw the fatal rock.

Legally, all three teenagers can be held convicted of causing Bartell’s death regardless of who threw the rock, both prosecutors and defense attorneys told jurors.

Koenig’s is the only case to go to trial. The other two teenagers in the car that night have pleaded guilty and are expected to testify during Koenig’s trial.

Nicholas “Mitch” Karol-Chik, 20, pleaded guilty to second-degree murder and attempted first-degree murder in 2024 and faces up to 72 years in prison. Zachary Kwak, 20, pleaded guilty days later to assault and attempted assault and faces between 20 and 32 years in prison.

Their sentencings are set for early May.