36
Opinion of the Court
out, we have done that case by case, largely avoiding categories and protocols for searches. Instead, we have treated reasonableness as a function of the facts of cases so various that no template is likely to produce sounder results than examining the totality of circumstances in a given case; it is too hard to invent categories without giving short shrift to details that turn out to be important in a given instance, and without inflating marginal ones. See, e. g., Ohio v. Robinette, 519 U. S. 33, 39 (1996) ("[W]e have consistently eschewed bright-line rules, instead emphasizing the fact-specific nature of the reasonableness inquiry"); Ker v. California, 374 U. S. 23, 33 (1963) (reasonableness not susceptible to Procrustean application); Go-Bart Importing Co. v. United States, 282 U. S. 344, 357 (1931) (no formula for determining reasonableness; each case on its own facts and circumstances). We have, however, pointed out factual considerations of unusual, albeit not dispositive, significance.
In Wilson v. Arkansas, 514 U. S. 927 (1995), we held that the common law knock-and-announce principle is one focus of the reasonableness enquiry; and we subsequently decided that although the standard generally requires the police to announce their intent to search before entering closed premises, the obligation gives way when officers "have a reasonable suspicion that knocking and announcing their presence, under the particular circumstances, would be dangerous or futile, or . . . would inhibit the effective investigation of the crime by, for example, allowing the destruction of evidence," Richards v. Wisconsin, 520 U. S. 385, 394 (1997). When a warrant applicant gives reasonable grounds to expect futility or to suspect that one or another such exigency already exists or will arise instantly upon knocking, a magistrate judge is acting within the Constitution to authorize a "no-knock" entry.2 And even when executing a warrant silent about
2 Some States give magistrate judges the authority to issue "no-knock" warrants, and some do not. See, e. g., Richards v. Wisconsin, 520 U. S. 385, 396, n. 7 (1997) (collecting state statutes and cases).
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