Teens accused of deadly rock-throwing spree formally charged with murder

Three Colorado teenagers arrested in connection with a rock-throwing spree that killed a driver have been formally charged with murder.

They also now face a dozen other counts for allegedly throwing large landscaping rocks toward seven vehicles on suburban Denver roads on April 19.

Joseph Koenig, Nicholas "Mitch" Karol-Chik and Zachary Kwak, all 18-year-old high school seniors, were arrested last month on first-degree murder, with extreme indifference, in connection with the rock-throwing spree.

The final rock-throwing incident killed 20-year-old driver Alexa Bartell. Three other people were injured, according to prosecutors.

The three teens have now formally been charged with one count of murder in the first degree, with extreme indifference, as well as six counts of criminal attempt to commit murder in the first degree, three counts of assault in the second degree and three counts of criminal attempt to commit assault in the second degree, the Colorado First Judicial District Attorney's Office announced Wednesday.

"Based on the available facts and after careful legal analysis, we have filed charges and will now move forward with a criminal prosecution," First Judicial District Attorney Alexis King said in a statement. "It is important to remember that these individuals are presumed innocent unless and until they are proven guilty. As in every case, our goal is to seek justice.”

Karol-Chik, Koenig, and Kwak are being held without bond. They appeared in court Wednesday for the return filing of charges.

The three defendants, dressed in orange jumpsuits and handcuffed, sat with their attorneys. A court official escorted six family members who sat in the front row. The families declined to talk to ABC News or identify themselves.

As the proceeding began, Judge Christopher Zenisek asked each defendant if they were aware of the charges. Their attorneys responded yes. The only words each defendant spoke were, “Yes, your honor.”

The next hearing in the case has been scheduled for May 16.

Koenig and Karol-Chik had allegedly thrown rocks at cars on at least 10 separate occasions since February, according to court documents.

Karol-Chik allegedly told police that on April 19, all three teens threw rocks at cars and they were "excited" when they hit them, according to court documents. He allegedly said Kwak was the one who threw the fatal rock at Bartell's car, the documents said.

After Bartell's car was hit, Kwak allegedly said, "We have to go back to see that," and when they turned around, Kwak allegedly took a photo of Bartell's car "as a memento," according to the documents.

ABC News' Emily Shapiro and Jeffrey Cook contributed to this report.