Golden, Colorado — Three Denver-area teenagers cheered one other on during a night of hurling rocks at automobiles, until one of the stones crashed through a windscreen and killed a woman, resulting in a murder conviction Friday after the trio turned on one another.

Jurors judged Joseph Koenig guilty of first-degree murder in the killing of Alexa Bartell on April 19, 2023, after the other young men riding with him negotiated an agreement with prosecutors and testified against him. Koenig, now 20, was also found guilty of attempted murder and other lesser offences for throwing rocks and other objects at vehicles on the night Bartell was killed and in the preceding weeks.

Bartell’s relatives and friends hugged and cried in court following the judgement.

Her mother, Kelly Bartell, later stated that justice had been served, but she had mixed sentiments, expressing sympathy for Koenig and the other two young men, all of whom were 18 at the time of her daughter’s death.

“It’s hard to be happy or feel satisfied that justice was served today, because I feel one amazing life was lost and three others are also lost and impacted,” according to her.

Throughout the two-week trial, jurors were forced to weigh shifting and contradictory versions of the truth presented by Koenig’s erstwhile co-defendants.

No one questioned that a 9-pound (4-kg) landscaping rock from a Walmart parking lot smashed through Bartell’s windscreen, killing her instantly. The problem was who tossed it. The only DNA found on the rock was Bartell’s, therefore the testimony of the other two, Zachary Kwak and Nicholas Karol-Chik, was crucial to the prosecution.

Lawyers representing Koenig claimed Kwak threw the rock that killed Bartell. But Kwak and Karol-Chik, whose plea deals on lesser counts potentially result in reduced prison sentences, claimed Koenig threw it. Although Karol-Chik claimed they each tossed around 10 pebbles that night, Kwak testified that he did not throw any.

Chief Deputy District Attorney Katharine Decker told jurors that the damage to Bartell’s car was consistent with Koenig — who is left-handed and was driving — throwing the rock out the driver’s-side window, as Karol-Chik claimed. Even if jurors were not sure that Koenig threw it, she said, they should still convict him of first-degree murder as a conspirator.

Koenig’s solicitors said he had no idea anyone had been injured until Bartell’s automobile drove off the road. They also claimed that he had borderline personality disorder, which impaired his impulse control and judgement.

Defence lawyer Martin Stuart asked jurors to convict Koenig of manslaughter, the least serious crime he faced, claiming he did not want to murder her. Jurors also had the possibility of convicting him of manslaughter as a conspiracy.

According to evidence, the three friends circled back many times to hunt for Bartell’s car after it had left the road. Kwak took a photo as a memory, but no one checked on the driver or requested assistance, according to their evidence.

Bartell’s body was not located until her girlfriend, Jenna Griggs, who was on the phone with her when it abruptly cut out, followed it down to the pitch, she testified.

The three agreed not to tell anyone about what happened, but Kwak, the group’s newest member, later informed authorities that Koenig threw the rock. Karol-Chik, who described Koenig as a “brother” to him, initially blamed Kwak before altering his narrative and blaming him.

According to Karol-Chik, Koenig appeared “excited” when they drove past Bartell’s automobile, making a “whoop” sound at one point.

“It sounded like him celebrating,” Karol-Chik claimed, admitting he placed the rock close to Koenig so he could grab and toss it.

Koenig’s lawyers attempted to cast doubt on the credibility of the other men’s testimony while emphasising that none of the three wanted to harm anyone. The defence declined to comment on the conviction.

Kwak first agreed to a plea deal, pleading guilty to first-degree assault in May 2024. In doing so, he admitted to acting in a way that put him at serious risk of death. He also pleaded guilty to second-degree assault and attempted second-degree assault for throwing rocks earlier that night. He faces 20 to 32 years in prison, according to authorities.

A week later, Karol-Chik pleaded guilty to second-degree murder and committing a crime of violence. He also pleaded guilty to attempted first-degree murder after throwing rocks at nine persons that night and earlier in 2023. According to his deal, Karol-Chik might face 35 to 72 years in prison when he is sentenced on Thursday, a day before Kwak.

Koenig is scheduled to be sentenced on June 3, and he faces a mandatory life sentence for murder.