Many agencies have jurisdiction to investigate financial crimes. An investigation might start with the Inspector General’s Office at an agency and from there, it may spread to the FBI. The IRS has jurisdiction over anything that involves tax violations.
When someone is alleged to have committed a financial crime or is alleged to have not declared ill-gotten gain on their tax return, the IRS has jurisdiction. The U.S. Postal Service may have jurisdiction when there is an allegation of a mail fraud scheme carried out through the postal service or a private mail carrier service.