26 USC 7606 - Entry of premises for examination of taxable objects

(a) Entry during day 
The Secretary may enter, in the daytime, any building or place where any articles or objects subject to tax are made, produced, or kept, so far as it may be necessary for the purpose of examining said articles or objects.
(b) Entry at night 
When such premises are open at night, the Secretary may enter them while so open, in the performance of his official duties.
(c) Penalties 
For penalty for refusal to permit entry or examination, see section 7342.