Police in San Bernardino, CA, raided a weed "fortress" on Wednesday, seizing 35,000 marijuana plants and shutting down an operation they believe was bringing in millions of dollars a month.

Investigators first turned their sights on the illegal grow operation about two months ago after receiving complaints from neighbors. They found the once-abandoned warehouse had been outfitted with a 12-foot metal rolling fence, "fortified doors," a large concrete wall around the parking lot and surveillance cameras, SCPR reports.

Inside the four-story warehouse, police found thousands of plants stacked next to one another under heat lamps on wood tables and an advanced irrigation system. The electric bill for the property was about $67,000 per month, police said.

"In my 26 years, it was the biggest grow that I've ever seen," San Bernardino police Lt. Mike Madden said. "There were all different rooms for different processes and hydration, filtration and ventilation. It was pretty extensive."

California voters have approved the legalization of marijuana, but growers must receive licenses and permits from local governments and the state. Recreational pot sales start in California on Jan. 1, joining the long-running medical cannabis industry.