The following excerpt is from U.S. v. So, 105 F.3d 667 (9th Cir. 1996):
The federal "knock and announce" statute, 18 U.S.C. 3109, 1 requires law enforcement officers executing a search warrant to identify themselves and state their purpose before forcibly entering a building. Absent exigent circumstances, forcible entry is justified only when there is an explicit refusal of admittance or a lapse of a significant amount of time. United States v. Mendonsa, 989 F.2d 366, 370 (9th Cir.1993).
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