Knock-and-announce, in United States law of criminal procedure, is an ancient common-law principle, incorporated into the Fourth Amendment, which often requires law enforcement officers to announce their presence and provide residents with an opportunity to open the door prior to a valid Fourth-Amendment search.
The rule is currently codified in the United States Code; Fourth-Amendment searches conducted by the federal government being governed thereby. Most states have similarly codified the rule into their own statutes, and remain free to interpret or augment the rule and its consequences in any fashion that remains consistent with Fourth-Amendment principles. A state's knock-and-announce rule will govern searches by state actors pursuant to a state-issued warrant, assuming that Federal actors are not extensively involved in the search.
In 1995, the Supreme Court ruled in Wilson v. Arkansas that a knock-and-announce before entry was a factor that must be considered in reviewing the overall constitutionality of a Fourth-Amendment search. After several state attempts to exclude specific categories (e.g. drug crimes) from the knock-and-announce rule based on blanket "factoring", the Supreme Court in Richards v. Wisconsin prohibited the policy, and demanded a return to a case-by-case review scenario. The Richards Court suggested that the knock and announce rule could be dispensed with only certain circumstances, for example where police have reasonable suspicion to believe that an exigent circumstance exists. The Court read its earlier Wilson opinion to suggest that such circumstances might include:
In a similar manner, where officers reasonably believe that exigent circumstances, such as the destruction of evidence or danger to officers will exist, a "no-knock" warrant may be issued. However, despite police awareness that such future exigencies will exist, they are generally not required to seek a "no-knock" warrant; in this case, police must have an objectively reasonable belief, at the time of executing the warrant, that such circumstances do in fact exist.
The Supreme Court has given some guidance as to how long officers must wait after knocking and announcing their presence before entry may be made. In U.S. v. Banks, the Supreme Court found 15 to 20 seconds to be a reasonable wait time where officers received no response after knocking and where officers feared the home occupant may be destroying the drug evidence targeted by the search warrant. As with most other things in the Fourth Amendment arena, the Court left reasonableness of the time period to be determined based on the totality of the circumstances; and thus inferior Federal courts have found even shorter time periods to be reasonable. Some different factors have been propounded by lower courts to guide the analysis of a reasonable wait period. A few examples are:
Federal courts also recognize that consent may vitiate part or all of the rule. For example, where officers knock, but before announcement are invited in, the need to announce is no longer required.
In Hudson v. Michigan (2006), the Supreme Court ruled that a violation of the knock-and-announce rule does not require the suppression of evidence using the exclusionary rule. This is primarily because the goals served by a knock-and-announce policy tend to be lesser than other requirements (such as the warrant requirement) of a valid Fourth-Amendment search: whereas the latter is to protect a reasonable expectation of privacy in a person's body, papers, and effects (among other things), the knock-and-announce rule is designed only to provide a brief moment of privacy for an individual to compose himself before a valid search occurs, to prevent an individual from mistakenly believing that police are common intruders and thus endangering them, and to prevent property damage from a forcible entry. Because police with probable cause and a valid warrant are already entitled to an entry and search, violation of the simple knock-and-announce rule has not been deemed grave enough in the Federal arena or in most states to justify suppression of the evidence.
Most states have composed their own statutes which require a knock and announcement before making a warranted entry. Because the states are free to offer more liberty to criminal defendants than the Federal constitution demands, the states remain free to impose the exclusionary rule for a violation of the knock-and-announce rule if they so wish. The Supreme Court opinion in Hudson is necessarily binding only on those searches conducted by the Federal government.