Mendocino County Sheriff Tom Allman said an internal investigation is under way to determine whether one of his captains was involved in growing marijuana on his family's property.

Sonoma County sheriff's officials said they are conducting the probe.

The investigation was launched after officers from the federal Drug Enforcement Administration, FBI and IRS earlier this month served a search warrant on property owned by Capt. Randy Johnson's father.

More than 500 plants were located during the federal raid, sources said.

Johnson owns property adjacent to his father. He, his father and his brother reside near each other on the properties. The wooded parcels along Highway 20 near Potter Valley also contain several rentals, including trailers. Some have enclosures shielded by high fencing.

Federal officials would not divulge details or the focus of their search.

Allman said he expects the internal investigation to be completed in a month. Johnson remains on duty.