Mendocino County Sheriff Tom Allman said Friday that an internal investigation has cleared a sheriff's captain of any potential wrongdoing related to a federal marijuana raid on property owned by the captain's family.

The Sonoma County Sheriff's Office conducted the investigation at Allman's request after federal authorities raided Potter Valley property owned by the family of Sheriff's Capt. Randy Johnson. Sources said authorities found 500 marijuana plants.

Allman refused to make the report public on Friday, saying it is a personnel matter. He said he was given permission to discuss the findings.

"I feel very comfortable in stating that Capt. Johnson was not involved in illegal activity," Allman said. "But if anyone can provide additional information the investigation will be re-opened."

Johnson was not immediately available for comment late Friday afternoon. He has remained at work during the inquiry.

Federal officials have not divulged any details about the case, which remains sealed under court order.

Allman said Sonoma County investigators found no evidence of marijuana growing in or around Johnson's house, which is on a 16-acre compound on Highway 20 that his family has owned for more than three decades.

One of the two parcels is owned by the captain and his father, Johnny Johnson. The adjoining parcel is owned by the elder Johnson and includes numerous rental residences.

Asked how anyone could have missed 500 marijuana plants being grown on the property, Allman said he doesn't know whether that number is accurate or whether the pot in question was being grown indoors or outside.

"Five-hundred plants doesn't mean that it looks like a vineyard," Allman said. "It could mean that the plants were inside. The DEA has not told me that Capt. Johnson is a suspect in this."

He said Sonoma County investigators concluded in a report that spans about 80 pages that Johnson was not involved in illegal activity and that Johnson was unaware of any such activity.

According to Allman, Johnson contacted state Department of Justice officials in 2010 and 2011 to report that he smelled marijuana at the property and he asked them to investigate. No action was taken, Allman said.

He said it was proper protocol for Johnson to reach out to state authorities, rather than his own department, to report his suspicions because the state oversees the sheriff's Major Crimes Task Force.

Allman said a renter who lives at the property told Sonoma County investigators that he had been growing marijuana at his home without Johnson's knowledge. The renter said federal authorities counted marijuana seedlings as plants.

Johnson's brother, John Johnson, also lives on the property. Allman said he declined to speak with investigators.

Randy Johnson was in charge of Mendocino County's previous medical marijuana growing permit program, which allowed cultivation of up to 90 plants,making it one of the more liberal regulations in the state. It was shut down after federal officials threatened to sue the county, which now allows 25 plants per parcel.

Allman said he's known Johnson for 28 years and that "his record is unblemished."

(You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com. On Twitter @deadlinederek.)