Brokerages, investors and banks are executing trades of dozens of marijuana firms listed on federally-supervised stock exchanges despite a dearth of official guidance on whether the transactions violate U.S. rules against money laundering. In August 2013, the U.S. Justice Department set out eight "priorities" for enforcing federal laws against marijuana amid several state ballot initiatives to legalize its use, and, in February 2014, advised that it "may not be appropriate" to investigate and prosecute financial institutions providing services that don't match those objectives. The U.S. Treasury Department's Financial Crimes Enforcement Network (FinCEN) concurrently issued its own guidance instructing banks to...