18 USC § 3109. Breaking doors or windows for entry or exit
The officer may break open any outer or inner door or window of a house, or any part of a house, or anything therein, to execute a search warrant, if, after notice of his authority and purpose, he is refused admittance or when necessary to liberate himself or a person aiding him in the execution of the warrant.
(June 25, 1948, ch. 645, 62 Stat. 820.)