26 U.S.C. § 6531: Periods of limitation on criminal prosecutions

for offenses involving the defrauding or attempting to defraud the United States or any agency thereof, whether by conspiracy or not, and in any manner; for the offense of willfully attempting in any manner to evade or defeat any tax or the payment thereof; for the offense of willfully aiding or assisting in, or procuring, counseling, or advising, the preparation or presentation under, or in connection with any matter arising under, the internal revenue laws, of a false or fraudulent return, affidavit, claim, or document (whether or not such falsity or fraud is with the knowledge or consent of the person authorized or required to present such return, affidavit, claim, or document); for the offense of willfully failing to pay any tax, or make any return (other than a return required under authority of part III of subchapter A of chapter 61) at the time or times required by law or regulations; for offenses described in sections 7206(1) and 7207 (relating to false statements and fraudulent documents); for the offense described in section 7212(a) (relating to intimidation of officers and employees of the United States); for offenses described in section 7214(a) committed by officers and employees of the United States; and for offenses arising under section 371 of Title 18 of the United States Code, where the object of the conspiracy is to attempt in any manner to evade or defeat any tax or the payment thereof.