unnecessary examination

If a taxpayer has filed fraudulent returns, a tax liability exists without regard to any period of limitations. Section 7602 authorizes the Commissioner to investigate any such liability. If, in order to determine the existence or nonexistence of fraud in the taxpayer's returns, information in the taxpayer's records is needed which is not already in the Commissioner's possession, we think the examination is not 'unnecessary' within the meaning of § 7605(b).

Defined by the Supreme Court in - United States v. Powell, 379 U.S. 48 (1964).