This is what I know the USSS (United states secret services) is under the agency of the U.S department of homeland security. Until 2003, the service was part of the U.S department of the treasury. This is also what I know the secret service is unique among the federal law enforcement agencies because not only do it's agents provide protection for the President, vice, former presidents, presidential candidates and visiting heads of state and heads of the government.
Hopefully this is helpful.