Joseph Koenig whooped with excitement after he threw a rock into a passing car’s windshield and killed the driver two years ago, his friend testified Friday during Koenig’s jury trial in Jefferson County District Court.

Koenig let out the whoop when he saw 20-year-old Alexa Bartell’s car had veered off the road after it was hit with the rock on Indiana Street near the Rocky Flats National Wildlife Refuge on April 19, 2023, Nicholas “Mitch” Karol-Chik testified Friday.

“He was excited,” Karol-Chik told jurors. “He would just look at us with this big smile on his face.”

Koenig is charged with first-degree murder in Bartell’s death, as well as attempted murder and related counts for his role in a rock-throwing spree that authorities say he, Karol-Chik and a third teenager, Zachary Kwak, carried out that night.

The teens threw landscaping rocks at a number of cars, both parked and moving, before hurling the rock through Bartell’s windshield in the last attack of the night, prosecutors allege.

All three men were 18 at the time of the rock-throwing spree.

Koenig’s celebration after hitting Bartell’s car was in line with how they’d reacted to past impacts, Karol-Chik testified.

The men would cheer and compliment each other whenever a rock struck a vehicle, he said. If they missed, they were disappointed, he said.

Koenig’s defense attorneys have conceded he helped to cause Bartell’s death and said that he should be held responsible. But they argued during opening statements that he was guilty not of first-degree murder, but of the less serious charge of manslaughter, because none of the teenagers intended to hurt anyone that night.

“We only thought of them as cars,” Karol-Chik testified Friday. “But we never thought of who may be driving them.”

Kwak and Karol-Chik have both pleaded guilty in Bartell’s killing, and, as part of their plea deals, are required to testify against Koenig.

Prosecutors say Koenig threw the fatal rock. Koenig’s defense attorneys say Kwak threw the fatal rock.

Following his arrest, Karol-Chik for months told investigators that Kwak threw the rock that killed Bartell, including through three proffer interviews in which he offered information to prosecutors in order to seek a plea agreement.

Not until a fourth and final proffer interview did Karol-Chik finally name Koenig as the person who threw the last rock, Karol-Chik admitted on cross-examination.

“You knew what you needed to say in this last interview,” Koenig’s defense attorney, Martin Stuart, said during cross-examination.

“I did,” Karol-Chik said.

“And that was that Joe threw the rock?” Stuart said.

“I knew that I had to tell them the truth,” Karol-Chik said.

“Well, you started out saying that you aren’t 100% sure that Joe threw the rock, and by the end of (the interview), you are?”

“That is correct,” Karol-Chik testified.

He testified Friday that he lied until the last interview to try to protect Koenig, who was at the time his best friend and “almost close enough to be brothers.”

Kwak was a newer friend who the pair had known for about a month, Karol-Chik testified.

He said he agreed to pin the crime on Kwak after talking with Koenig after Bartell’s death.

“We went over everything that happened and it was at that time that Joe said to me that we wouldn’t have to worry about it, we’ll just say that Zach did it,” Karol-Chik testified.

But under cross-examination, Karol-Chik testified he believed Kwak threw the fatal rock and that he thought he remembered Kwak handling that particular rock, which was bigger than the other rocks they’d collected that night.

He testified that he told Kwak not to use that rock, because it was too big, but that Kwak told him that he would throw it if Karol-Chik would not.

The rock weighed 9.3 pounds, testified Dan Manka, an investigator with the Jefferson County Sheriff’s Office.

Stuart suggested Karol-Chik pointed a finger at Koenig only to secure a plea deal with the district attorney’s office that would allow him to avoid a life prison sentence, and that he tailored his testimony to match the prosecution’s theory of events after prosecutors offered no deal after the first three interviews in which he named Kwak as the person who threw the fatal rock.

Karol-Chik’s testimony Friday followed Kwak’s testimony Thursday and differed on several details. Kwak said he didn’t throw any rocks during the several hours that he hung out with the other teenagers that night and had asked to go home; Karol-Chik said all three teenagers threw the rocks and that Kwak never asked to leave.

Karol-Chik also said Koenig was driving at about 80 mph when he threw the rock that killed Bartell, while Kwak said the speed was 103 mph. Both testified that Koenig sped up before the fatal throw.

The two men’s testimony aligned on the major events of the night, including that they carried out a series of escalating pranks and rock-throwing before Bartell was killed.

After her death, Karol-Chik said he knew they’d “(expletived) up” when he drove by the area again later that night and saw emergency vehicles, he testified.