Because of its role in investigating counterfeiters, the new Secret Service was created under the United States Department of the Treasury. The Service began in earnest, and by 1866 agents had already shut down some 200 counterfeiting operations working in the United States.
So successful were they, in fact, that in 1867 their authority was expanded.
Counterfeiters were no longer their only target; they were authorized to
investigate any persons committing fraud against the government.
Secret Service, the only other real federal police force at this time were the
U.S. Marshals. The Secret Service soon began taking over many of the
functions in enforcing federal law that the Marshals did not have the manpower
to handle on their own.