Knock and announce Quote: Richards

 In Wilson v. Arkansas, 514 U.S. 927 (1995), we held that the Fourth Amendment incorporates the common law requirement that police officers entering a dwelling must knock on the door and announce their identity and purpose before attempting forcible entry. 

[. . .]

In order to justify a "no knock" entry, the police must have a reasonable suspicion that knocking and announcing their presence, under the particular circumstances, would be dangerous or futile, or that it would inhibit the effective investigation of the crime by, for example, allowing the destruction of evidence. This standard--as opposed to a probable cause requirement--strikes the appropriate balance between the legitimate law enforcement concerns at issue in the execution of search warrants and the individual privacy interests affected by no knock entries. Cf. Maryland v. Buie, 494 U.S. 325, 337 (1990) (allowing a protective sweep of a house during an arrest where the officers have "a reasonable belief based on specific and articulable facts that the area to be swept harbors an individual posing a danger to those on the arrest scene"); Terry v. Ohio, 392 U.S. 1, 30 (1968) (requiring a reasonable and articulable suspicion of danger to justify a pat down search). This showing is not high, but the police should be required to make it whenever the reasonableness of a no knock entry is challenged.

Richards v. Wisconsin, 520 US 385 (1997)