Twelve Hamilton residents were arrested Thursday morning for what RCMP is calling "egregious exploitation" of Canada's medical marijuana regulations.

Police say a profitable marijuana grow-op was disguised as a medical marijuana operation that had licences to cultivate plants for other people.

"Our allegations are they abused the MMAR [Health Canada's Medical Marihuana Access Regulations system] and, through other planning, developed access to licences to profit from the marijuana," said Inspector Steve Martin of the RCMP.

The operation was uncovered as part of an 18-month investigation that included undercover officers embedded in organized crime groups.

The RCMP and Hamilton Police said they have arrested four members of the same family, the Gravelles, as well as their lawyer and other co-accused.

According to police, licences for medical marijuana production were obtained fraudulently. They allege that a distribution network extended as far east as Newfoundland.

At a press conference Thursday afternoon, police said they'd seized 240 mature marijuana plants that were being grown in trailers at two addresses on Beech Avenue in Hamilton. They added that it was difficult to estimate a street value for the marijuana.

The accused face a number of charges, including participation in a criminal organization, possession for the purpose of trafficking a controlled substance and trafficking a controlled substance. All were released on their own recognizance.

The accused are scheduled to appear in Hamilton court in April. The date of the appearance has not yet been set.