27 C.F.R. § 70.30
(b) Restrictions on examination of taxpayer. No taxpayer is to be subjected to unnecessary examination or investigations, and only one inspection of a taxpayer's books of account shall be made for each taxable year unless the taxpayer requests otherwise or unless an authorized internal revenue or an appropriate TTB officer, after investigation, notifies the taxpayer in writing that an additional inspection is necessary.
(68A Stat. 902, as amended (26 U.S.C. 7605))
[T.D. ATF-450, 66 FR 29023, May 29, 2001]