By Brett Wolf ST. LOUIS (Reuters) – The U.S. Treasury Departments anti money-laundering unit is warning businesses linked to the digital currency Bitcoin that they may have to comply with federal law and regulation as money transmitters, a Treasury spokesman said. Treasurys Financial Crimes Enforcement Network (FinCEN) has sent industry outreach letters to about a dozen firms, regarding potential anti-money laundering compliance obligations related to Bitcoin businesses, FinCEN spokesman Steve Hudak told Thomson Reuters regulatory information service Compliance Complete.