The Colorado teenager accused of throwing a rock that killed a passing driver two years ago was diagnosed with mental conditions that impacted his judgment and impulse control at the time of the fatal attack, a forensic psychiatrist testified Tuesday.
Joseph Koenig, who is now 20 but was 18 at the time of the Jefferson County rock-throwing spree, is standing trial on charges of first-degree murder, nine counts of attempted murder, three counts of assault and six counts of attempted assault.
He is accused of throwing a 9.3-pound rock into the windshield of a passing car on April 19, 2023, killing the driver, 20-year-old Alexa Bartell. The fatal attack was one of several times Koenig and two other teenagers threw rocks at cars that night.
Forensic psychiatrist Alexandra Lynch diagnosed Koenig with attention-deficit hyperactivity disorder, or ADHD, as well as borderline personality disorder after completing a court-ordered mental health evaluation of Koenig.
Both conditions impacted his judgment at the time of the rock-throwing spree, she testified for the defense Tuesday. Borderline personality disorder is characterized by chronic feelings of emptiness and is often associated with reckless behavior, unstable relationships, particularly with peers, as well as self-harm and impulsivity. ADHD is characterized by inattention, hyperactivity or impulsivity.
“His impulse control, decision-making and judgment were impacted by both of those diagnoses,” she testified.
On cross-examination, Lynch admitted that borderline personality disorder does not stop someone from knowing right from wrong or understanding the consequences of actions.
Koenig’s attorneys have presented his actions during the 2023 rock-throwing spree as thoughtless teenage behavior, rather than first-degree murder. They admitted he is responsible for Bartell’s death, but argued he should be found guilty of the lesser charge of manslaughter. First-degree murder carries a mandatory life prison sentence.
The defense case opened Tuesday with a focus on Koenig’s mental health and teenage brain development after prosecutors with the Jefferson County District Attorney’s Office rested their case late Friday. The trial did not take place on Monday.
Laurence Steinberg, a at Temple University, testified as an expert for the defense Tuesday that young people’s brains continue to develop into their early 20s, and that adolescent brains differ from adult brains in several key ways.
Adolescents are more impulsive than adults, more focused on the rewards and short-term consequences of decisions than the downsides and long-term consequences, and are more susceptible to the influence of peers than adults, Steinberg testified.
“Adolescence is a time when the accelerator in the brain is pushed down to the floor — that is the limbic system — but there is not yet a good braking system in place. That is the prefrontal cortex,” Steinberg testified.
Research shows that people between the ages of 18 and 21 can at times think and act like adults, but are more likely to think and act immaturely, like younger teens, when they are experiencing heightened emotions and when they are with their peers, Steinberg testified.
The atmosphere inside the truck on the night of the fatal rock-throwing spree — in which then-18-year-olds Nicholas “Mitch” Karol-Chik and Zachary Kwak were cheering and encouraging each other to throw rocks — would create that heightened emotional state and likely compromise the teenagers’ decision-making, Steinberg testified.
On cross-examination, the professor admitted that individual brain development differs and some 18-year-olds are more mature than others. He also acknowledged that research on the brain has evolved over the last couple of decades and that some studies have contradicted some findings on adolescent brains.
The defense expert’s testimony follows testimony from the two other men who were in the car that night, who both pleaded guilty and agreed to testify against Koenig as part of their plea agreements.
Karol-Chik pleaded guilty to second-degree murder and attempted first-degree murder in 2024 and faces up to 72 years in prison. Kwak pleaded guilty days later to assault and attempted assault and faces between 20 and 32 years in prison.
Both are scheduled to be sentenced in early May.
Koenig’s trial will continue Wednesday.
