A tree worker was killed Thursday while working on the Butte Fire Tree Removal Project in Mountain Ranch, according to the Calaveras County public information officer.

21-year old Chris Hiser of Sonora was killed when an unknown accident occurred, officials say.

The accident took place on Whiskey Side Road on the site of the Tree Removal Project for the County and FEMA.

An investigation is underway to determine the exact cause of death.