STAPLES (AP) - The whereabouts of a northern Minnesota man remain a mystery four years after he disappeared.

Eighty-two-year-old Peter Achermann of rural Staples left his home to run errands on July 24, 2009. He was last seen on Warner Road by the Crow Wing River Bridge between noon and 1 p.m.

Achermann's car was found the next day stuck in a mudhole on a dead-end road about six miles northeast of Staples. But despite extensive searches, no trace of Achermann has ever been found.

Cass County Sheriff Tom Burch says his office is working with the Minnesota Bureau of Criminal Apprehension and is committed to solving the case.

A reward of up to $20,000 has been extended for information leading to the arrest and conviction of anyone responsible for Achermann's disappearance.