Three Denver-area teenagers cheered each other during a night of throwing rocks at cars — until one of the stones crashed through a windshield and killed a woman, leading to a murder conviction Friday after the trio turned on one another.

Jurors found Joseph Koenig guilty of first-degree murder in the death of Alexa Bartell on April 19, 2023, after the other young men riding with him reached deals with prosecutors and testified against him. Koenig, now 20, was also convicted of attempted murder and other less serious crimes for rocks and other objects thrown at vehicles the night Bartell was killed and in previous weeks.

Bartell’s family and friends hugged and cried in court after the verdict.

Her mother, Kelly Bartell, said later that justice had been done but she had mixed feelings, expressing some sympathy for Koenig and the other two young men, who were all 18 when her daughter was killed.

“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,” she said.

Jurors had to consider shifting and competing versions of the truth offered by Koenig’s former co-defendants during the two-week trial.

Kelly Bartell, the mother of Alexa Bartell who died after a rock crashed through the windshield of her car, speaks at Jefferson Combined Court in Golden, Colo., on Friday.  (AP Photo/Colleen Slevin)

No one disputed that a 9-pound (4-kilogram) landscaping rock taken from a Walmart parking lot crashed through Bartell’s windshield, killing her instantly. The issue was who threw it. The only DNA found on the rock was Bartell’s, making the testimony from the other two, Zachary Kwak and Nicholas Karol-Chik, key to the prosecution.

Lawyers for Koenig said Kwak threw the rock that killed Bartell. But Kwak and Karol-Chik, whose plea agreements on lesser charges could lead to shorter prison sentences, said Koenig threw it. Although Karol-Chik said they each threw about 10 rocks that night, Kwak testified that he did not throw any.

Chief Deputy District Attorney Katharine Decker told jurors the damage to Bartell’s car was consistent with Koenig — who is left-handed and was driving — throwing the rock, shotput-style, out the driver’s-side window, as Karol-Chik testified. Even if jurors were unconvinced that Koenig threw it, she told them, they should still find him guilty of first-degree murder as a conspirator.

Koenig’s attorneys said he did not know anyone had been hurt until Bartell’s car went off the road. They also argued that he had borderline personality disorder, affecting his impulse control and judgment.

Defense lawyer Martin Stuart asked jurors to instead find Koenig guilty of manslaughter, the least serious charge he faced, saying he did not knowingly try to kill her. Jurors also had the option of finding him guilty of manslaughter as a conspirator.

After seeing Bartell’s car leave the road, the three friends circled back a few times to look again, according to testimony. Kwak took a photo as a memento, but no one checked on the driver or called for help, according to their testimony.

Bartell’s body would not be discovered until her girlfriend, Jenna Griggs, who was on a call with her when it abruptly cut out, tracked her phone to the field, she testified.

The three agreed not to talk to anyone about what happened, but Kwak, the newest to the group of friends, later told investigators that Koenig threw the rock. Karol-Chik, who said Koenig was like a “brother” to him, initially pointed the finger at Kwak before changing his story and blaming Koenig.

Karol-Chik testified that Koenig seemed “excited” as they drove by Bartell’s car and at one point made a “whoop” sound.

“It sounded like him celebrating,” said Karol-Chik, who admitted placing the rock next to Koenig so he could grab it and throw it.

Koenig’s lawyers tried to cast doubt on the reliability of the other men’s accounts but also stressed that none of the three intended to hurt anyone. The defense declined to comment on the conviction.

Kwak entered into a plea deal first, pleading guilty in May 2024 to first-degree assault. In doing so he acknowledged acting in a way that created a grave risk of death. He also pleaded guilty to second-degree assault and attempted second-degree assault for rocks that were thrown earlier in the night. He faces between 20 and 32 years in prison, according to prosecutors.

About 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 for throwing rocks at a total of nine people that night and earlier in 2023. Under his agreement, Karol-Chik could be sent to prison for between 35 and 72 years when he is sentenced Thursday, a day before Kwak.

Koenig is to be sentenced June 3 and faces a mandatory life term for the murder conviction.