Earlier quoted context omitted.
Isn't the Secret Service supposed to protect the president?
They were a part of the Treasury Department until 2003 (now DHS) and deal with counterfeiting and other financial crimes. I don't know for sure that fraud like this would land in the Secret Service queue and not the FBI but it's certainly plausible. It mostly depends on which specific statute(s) you're being investigated for as to which agency(ies) do the investigating.
Used to be; but now they are part of Homeland Security.