Mendocino County authorities discovered a marijuana extraction laboratory in a Willits industrial building where an explosion blew a large hole in the roof, the Sheriff’s Office said Friday.

Firefighters with the Little Lake Fire Protection District responded to an explosion about 1:40 p.m. Tuesday at a 30,000-square-foot, metal-sided building in the 500 block of Cropley Lane, Capt. Greg Van Patten said in a press release.

There was no sign of a fire, but officials found a hole in the roof caused by an “unknown projectile,” he said.

No one was found in the building, but deputies discovered the lab inside and several greenhouses on the property that contained several hundred marijuana plants, Van Patten said. Investigators determined a cannabis cultivation permit had been issued by the Mendocino County Agriculture Department for the property.

An investigation by agents with the Mendocino County Major Crimes Task Force is ongoing, Van Patten said. Investigators asked anyone with information on the incident to call the Sheriff’s Office Tip Line at 707-234-2100.