Under the common law knock-and-announce rule, a police officer executing a search warrant generally must not immediately force his or her way into a residence. Instead, the officer must first knock, identify himself or herself and his or her intent, and wait a reasonable amount of time for the occupants to let him or her into the residence.
When a lawsuit is filed over an officer's entry into a home, a court conducts an inquiry into whether the entry was reasonable. In Wilson v. Arkansas, the Supreme Court held that whether the knock-and-announce principle was adhered to constitutes one factor that a court must consider.
Permissible No-Knock Situations
In Richards v. Wisconsin, the Supreme Court held that an officer is not required to knock and announce if doing so would be unreasonable. This includes situations in which officers suspect that announcing their presence would be dangerous, futile, or result in the destruction of evidence.
For more on the knock-and-announce rule, see this Northwestern University Journal of Criminal Law and Criminology article, this University of San Francisco Law Review article, and this Missouri Law Review article.