Nicole Bergeron had responded to countless emergencies in two decades as a firefighter and paramedic. But nothing could prepare her for the call when Alexa Bartell was killed by a rock thrown through her windshield.

The last of three friends accused of killing a driver in Colorado by throwing a rock through the windshield of her car was convicted of first-degree murder Friday. The teens were driving around suburban Denver throwing rocks at passing cars when one struck and killed 20-year-old Alexa Bartell, in a crime prosecutors classified as murder.

Joseph Koenig was found guilty of all charges related to a fatal rock-throwing incident involving Alexa Bartell. His co-defendants testified against him as part of plea deals.