A massive police investigation has resulted in the arrests of a dozen more people, six years after a massive marijuana grow-op was uncovered running out of a brewery in Barrie, Ont.

Ontario Provincial Police announced on Thursday that officers had arrested 12 people from Ontario and Quebec and seized property and vehicles worth more than $7.5 million.

Det.-Insp. Andy Karski said they executed five search warrants on homes and industrial complexes on Wednesday.

"Police in Quebec have also simultaneously executed search warrant targeting a criminal enterprise involved in large-scale marijuana production," Karski told a media conference.

The properties police seized on Wednesday include three vehicles, property in Elliott Lake, Ont., and a building formerly used as a Molson brewery.

Police swarmed the brewery in January 2004, and seized 30,000 marijuana plants, with a street value of about $30 million. Nine people were arrested at the time.

At the time it was the largest marijuana grow operation discovered in Canada.

On Thursday, OPP said a massive investigation led them to charge a dozen more people, and accused them of being the masterminds behind the large-scale operation.

The suspects are charged with drug and criminal enterprise offences.

"Marijuana grow operations pose a real threat to public and police safety," OPP Supt. Dan Redmond told reporters at a press conference on Thursday.

Redmond said the majority of marijuana grown in Canada is sent to the U.S. in exchange for cocaine and guns.