18 U.S.C. § 3109 : US Code - Section 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.