This requirement was clarified in the 1995 Supreme Court case Wilson v. Arkansas in which the Court unanimously held that the common law knock-and-announce "principle forms a part of the reasonableness inquiry under the Fourth Amendment." Therefore, "the reasonableness of a search of a dwelling may depend in part on whether law enforcement officers announced their presence and authority prior to entering."
Proponents of the knock-and-announce rule assert that 1) it protects life and limb because a resident may mistake an unannounced police entry for a criminal invasion and brandish a weapon in defense; 2) it protects property by giving the occupant the opportunity to admit the officers; 3) and it protects privacy and preserves dignity by reducing the risk that the police will enter the wrong residence and even when there was no mistake about the place to be searched, permits those within to have a brief notice to prepare for the police entry. For example, if officers knock and announce their presence, a nude resident might have time to don a robe before officers enter his bedroom.
Even when the entry is made without force such as when the door to the residence is not locked or an apartment manager agrees to open the door, you must ordinarily observe the requirement to knock and announce before entering. Some courts have even ruled that you must first announce your presence before entering a wide open door.[PAGEBREAK]
To comply with the notice requirements under the knock-and-announce rule, you must identify yourselves as police and indicate that you are at the home to execute a search warrant. After this announcement is made, you must wait a reasonable time for an occupant of the premises to admit you. If you are denied entry or there is no response from within the residence, you may proceed with a forcible entry if necessary.
The Supreme Court first addressed how much delay is necessary before forcible entry in United States v. Banks. They concluded that a 15- to 20-second wait by the police before they forced the door open with a battering ram was sufficient because the police had reason to believe that if they waited longer, Banks would destroy his suspected stash of cocaine. The officers that executed the search were unaware that Banks was showering when they knocked on the door. The Court reasoned, "[T]he facts known to the police are what count in judging reasonable waiting time."