The federal Drug Enforcement Administration, FBI and IRS are conducting a criminal investigation of family members of a Mendocino County sheriff's captain, according to law enforcement sources.

Officials on Thursday served a search warrant on property near Highway 20 in Potter Valley where agents reportedly found more than 500 marijuana plants. The property is owned by and is the residence of Johnny Johnson, the father of Sheriff's Capt. Randy Johnson. The captain's brother, John Johnson, also lives on the property.

Randy Johnson owns and lives on property adjacent to his father's, according to county records.

None of the Johnsons could be reached Monday for comment. Randy Johnson was at work Monday but did not return phone calls.

DEA spokeswoman Casey Rettig would confirm only that the agency's agents served a search warrant in the Potter Valley area. FBI and IRS officials confirmed they were involved but also would not divulge any details.

The search warrant is sealed, Rettig said.

Sheriff Tom Allman said he'd heard about the search warrant being served but said he had no other information because federal authorities did not contact him.

"I've never been told what happened," he said.

Allman said he has not discussed the incident with Johnson. He said he has no reason to believe Johnson knew about the pot plants.

"It would be a concern if any of my employees knew about that many plants" and didn't report them, Allman said.

Mendocino County's per-parcel limit for marijuana plants is now 25.

Randy Johnson has been something of an advocate for medicinal marijuana growing since he was put in charge of the county's now-defunct marijuana growing permit program. The program allowed up to 99 plants with a permit. It was shut down after federal authorities threatened to sue the county.

Johnson also lobbied Lake County officials to adopt medicinal marijuana permits to generate revenue and testified on behalf of men arrested in Sonoma County for transporting medicinal marijuana from Mendocino County to patients in the Bay Area.

You can reach Staff Writer Glenda Anderson at 462-6473 or glenda.anderson@pressdemocrat.com.