Under the knock-and-announce rule, law-enforcement officers, before searching a residence, are required, 1) to announce the officers' presence and 2) to provide residents an opportunity to open the door.
The constitutional purpose of the knock-and-announce rule is to prevent needless destruction of property (such as breaking down a door) and to avoid unpleasant surprise. The Fourth Amendment protects the constitutional right of the people to be free from unreasonable searches and seizures. The knock-and-announce rule is one of several means intended to protect this right. An officer may break open any outer or inner door or window of a house, or any part of a house, or anything therein, in order to execute a search warrant, if, after notice of his authority and purpose, he is refused admittance or when necessary to liberate himself or a person aiding him in the execution of the warrant.