The United States Secret Service (USSS or Secret Service) is a federal law enforcement agency under the Department of Homeland Security tasked with conducting criminal investigations and providing protection to U.S. political leaders, their families, and visiting heads of state or government. The Secret Service was, until 2003, part of the Department of the Treasury, due to their initial mandate of combating counterfeiting of U.S. currency. The agency has protected U.S. presidents and presidential candidates since 1901.