GOLDEN, Colo. (KDVR) New details were revealed Thursday by a key witness in a fatal rock-throwing case in Colorado.

That witness was one of three teenagers who were in a truck from which a rock was thrown that killed a 20-year-old innocent driver in Jefferson County.

The witness walked jurors through the night Alexa Bartell was killed. Twenty-year-old Zachary Kwak described collecting landscaping rocks with his friends and targeting both parked and moving cars.

The jury has been listening to testimony in the trial of Joseph Koenig since Monday. Koenig is charged with multiple charges, including first-degree murder, for allegedly throwing the rock that killed Bartell.

The 20-year-old woman was struck in the head and killed as she was driving her vehicle in a sparsely populated area in the 10-600 block of Indiana Street in Jefferson County in April 2023.

There were three 18-year-old high school students in the truck from which rocks were being thrown. Kwak testified that he never threw rocks himself.

Kwak said Koenig would make a “whooping” sound and was excited as he drove the truck and threw rocks out of the window. Kwak said some of the rocks were as large as softballs. Those rocks were shown as evidence in the trial on Thursday.

Bartell’s family members became emotional when Kwak described the truck moving at 103 mph and throwing the rock that killed Bartell.

“I remember hearing the last one. Somebody said ‘Last one.’ Indicating to me this was the last one before the night. I remember hearing ‘Faster. Faster.’ I remember hearing the noise that the window makes on the highway,” Kwak said.

Kwak said the rock made a loud boom. Kwak then described turning his head around and seeing Bartell’s car go off the road slowly.

He described seeing Bartell in the wrecked car later and taking a picture, but none of them tried to help Bartell, who had been fatally wounded. Kwak told Koenig’s defense attorney he never actually saw the stone that killed Bartell.

Instead, Kwak said, he only saw the outline.

Kwak has pleaded guilty to assault charges and agreed to testify. Another defendant, Nicholas “Mitch” Karol-Chik, is also expected to testify. Karol-Chik has also pleaded guilty and agreed to testify as part of the plea agreement.

The trial is scheduled to resume Friday morning.