Jury selection in the homicide trial of a man accused of throwing a landscaping rock into oncoming highway traffic in Westminster, killing a 20-year-old woman, is slated to begin Friday morning.

Joseph Koenig, who was 18 at the time of his arrest, faces charges of first-degree murder, attempted murder, assault and attempted assault in connection with deadly incident in April 2023.

Koenig, along with friends Zachary Kwak and Nicholas “Mitch” Karol-Chik, also 18 at the time of their arrests, allegedly threw several rocks, a concrete statue head and a vehicle fender from a vehicle onto oncoming highway traffic.

The three allegedly hit several cars in under 45 minutes between 10 p.m. and 11 p.m. on April 19, 2023, inflicting damages on them while driving around in a truck belonging to Karol-Chik.

Then, at 10:45 p.m., they allegedly hurled a large, three- to five-pound landscaping rock into oncoming traffic, instantly killing 20-year-old Alexa Bartell.

Bartell was talking on the phone with a friend when the line fell silent. Investigators believe it was at that moment when the rock crashed through her windshield and into her head.

Jefferson County Sheriff’s investigators testified in prior court hearings Kwak told them that the three teens made a U-turn, driving by Bartell’s car to survey the damage they had caused. Kwak also told police he took a photo her vehicle for a memento.

While the truck belonged to Karol-Chik, Koenig was allegedly driving and Kwak was sitting in back, handing rocks up front to be thrown, according to arrest records and court testimony.

Each of the three suspects were initially charged with first-degree murder, but Kwak and Karol-Chik both pleaded guilty to less severe charges in plea deals and are expected to testify against Koenig during the trial. The two will be sentenced after Koenig’s trial is over.

Last July, Koenig’s defense attorneys argued that an ADHD diagnosis qualified him for an insanity defense.

Also of note is whether the three had ever thrown objects at cars before. Karol-Chik told investigators that he and Koenig threw objects at oncoming cars on at least 10 separate nights, but defense attorneys said that his statement was vague and lacked detail, according to a motion obtained by The Denver Gazette.

First Judicial District Judge Christopher Zenisek ruled in February that this information from Karol-Chik’s video-taped interview can be admitted as evidence at the trial, but only during the prosecution’s rebuttal of a psychiatric defense witness.

Dr. Lawrence Steinberg, an expert in adolescent brain development, is reportedly expected to testify in the trial that Koenig only threw the large rock because he was experiencing a feeling of “hot cognition,” a state of mind when emotions control the thought process, resulting in risky behavior.

But depending on how many incidents of rock throwing there were, with potentially weeks in between them, prosecutors have argued that Koenig must have had time to think about the consequences of his actions, and advised the court that forensic psychiatrist Alexandra Lynch would be testifying for its side.

The trial is expected to last about two weeks, according to a spokesperson for the District Attorney’s office. The public will be able to watch a live video feed on Webex under Jefferson County courtroom 520. Denver7 and 9News have been permitted to record and rebroadcast audio and video of the trial through the livestream.

The parents of all three defendants, as well as Bartell’s family and friends, have been present in the courtroom during the preliminary process and are expected to attend the trial.

Bartell’s parents have not missed a court date and have opted not to speak to the press.

Former Denver Gazette reporter Carol McKinley contributed to this story.