Most government agencies derive their investigative authority from the legislation that created them. For instance, Title 28, Section 533 of the United States Code gives the FBI the authority to investigate any federal crime, which is not under the exclusive authority of another department.
Due to limited Federal resources, a defendant will not normally be prosecuted for mail and wire fraud if the scheme is small in scale, causes only minor loss, and involves transactions between only a few individuals. Thus, if a person pays for an expensive set of tires with a forged check that he knows will not clear the bank, then disappears, most likely a federal investigation will not be initiated with regard to the fraud. However, any scheme that by its nature is directed at a large class of people or the general public, and involves a substantial pattern of conduct, is more likely to lead to prosecution under the mail and wire fraud statutes. The large scale of the fraud is likely to invite federal investigative attention, and in getting to the bottom of the scheme investigators are likely to come across several instances of mail and wire fraud.
The Criminal Resources Manual for the United States Department of Justice dictates consultation requirements for prior to the initiation of some investigations. For example, in an election fraud investigation, the FBI is required to consult with the Public Integrity Section of the Department of Justice before bringing mail and wire fraud charges in an indictment. Similarly, in a tax evasion case, the prosecuting authority is required to consult with the Department of Revenue before adding charges of mail or wire fraud to the tax charges.