LOS ANGELES -- Police in Southern California 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 neighbours. 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.

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."

Police and federal investigators raided three properties owned by the same woman and seized 18,000 pounds (8,164 kilograms) of marijuana in total. Eight people who were working in the warehouse in downtown San Bernardino were detained by police, but no charges have been filed in the case, Madden said.

Investigators are still trying to determine where the marijuana was being sold, Madden said.

Police were investigating the owner of the properties, Stephanie Smith, 43, but she was not arrested or charged with a crime. A telephone number for Smith could not be located in public records.

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.

"Marijuana has been legalized, but there are stringent requirements," Madden said. "It's not that you just get to set up shop where you want to set up."