the police knew they had reliable information that cocaine was being sold at that location. Faced with the probable imminent destruction of evidence, the police acted properly by entering the premises at once. To require the police in these circumstances to announce that they are there to execute a search warrant would be to require a futile act. See, e.g., United States v. White, 514 F.2d 205, 207 (D.C.Cir.1975); United States v. Wylie, 462 F.2d 1178, 1186-87 (D.C.Cir.1972). Compliance with section 3109 is unnecessary in such circumstances. See, e.g., United States v. Smith, 520 F.2d 74, 80-81 (D.C.Cir.1975); Masiello v. United States, 317 F.2d 121, 122-23 (D.C.Cir.1963); see also Sabbath v. United States, 391 U.S. 585, 591 & n. 8, 88 S.Ct. 1755, 1759 & n. 8, 20 L.Ed.2d 828 (1968) (<HOLDING>); Ker v. California, 374 U.S. 23, 39-41, 83
recognizing exceptions to immunity recognizing that exigent circumstances exceptions by their nature turn upon the objective reasonableness of ad hoc factspecific assessments recognizing first two exceptions recognizing that exceptions to section 3109 may apply in exigent circumstances holding that in the absence of exigent circumstances the police may not enter a private residence to make a routine arrest