Who does Forsberg blame for the destruction of the cabins?

Forsberg believes that McCandless was responsible for the destruction of his cabin.

Forsberg considers McCandless the prime suspect. He believes McCandless blundered upon the cabins after arriving at the bus during the first week of May, flew into a rage over the intrusion of civilization on his precious wilderness experience, and systematically wrecked the buildings. This theory fails to explain, however, why McCandless didn’t, then, also trash the bus.