The Internal Revenue Service has a Criminal Investigation Division, the sole purpose of which is to put people in jail. They will investigate leads from various sources, and recommend certain cases for criminal prosecution by the U.S. Justice Department. Their conviction rate is very high, and the appearance of an IRS Special Agent is extremely serious.
Under no circumstances should you talk to the CID—just take their business card, close the door, and call your attorney. Everyone thinks he or she is innocent; but the IRS is not paid to believe you!