@Telesyst - Regulatory agencies are more likely to be in charge of making the rules and laws. However, regulatory authorities, like the Internal Revenue Service or Securities and Exchange Commission, for example, can investigate and charge companies and individuals with violations.
A large part of the IRS' work is actually investigating tax and securities fraud.
If you are charged with any violations of tax or securities laws, the penalties these agencies hand down are very real and enforceable.
Both of these agencies sometimes work with U.S. attorneys or even the Federal Bureau of Investigation, depending on the crime.