18 U.S. Code § 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.
Based on title 18, U.S.C., 1940 ed., §§ 618, 619 (June 15, 1917, ch. 30, title XI, §§ 8, 9, 40 Stat. 229).
Said sections 618 and 619 were consolidated with minor changes in phraseology but without change of substance.