The authorities are pressuring landlords to shut down the shops or face possible loss of the real estate through the unconventional and low-key use of a civil statute designed primarily to seize the assets of drug-trafficking organizations. While some states have legalized medical marijuana businesses, the federal government does not recognize their authority to do so and has targeted the shops for violations of the 40-year-old Controlled Substances Act. The goal of the Justice Department's effort is to fight the medical marijuana industry, estimated at $1.7 billion annually, without confronting it head-on with costly and potentially embarrassing criminal prosecutions.