Cowlitz County Coroner Tim Davidson said Friday that dental records were used to determine the skull belonged to Sam Stefonek.
The Daily News reports that Davidson said the skull showed no sign of trauma, and the cause of Stefonek's death remains a mystery.
The Kelso man disappeared Aug. 8, 2010. Authorities said he argued with his girlfriend, then fled into the woods behind his parents' home. His cell phone was found nearby.
After multiple previous searches, about 40 volunteers returned to the area last Saturday. Dogs trained to find cadavers led searchers to the skull.
Cowlitz County sheriff's spokesman Charlie Rosenzweig says investigators will have a hard time determining how Stefonek died unless more of his remains are found.