Police officers obtain search warrants by convincing a judge or magistrate that they have "probable cause" to believe that criminal activity is occurring at the place to be searched or that evidence of a crime may be found there. If the magistrate believes there is probable cause to conduct a search, he or she will issue a warrant. The suspect can later challenge the validity of the warrant before trial.
A search warrant is an order signed by a judge that authorizes police officers to search for particular objects or materials at a specified location and time. For example, a warrant may authorize the search of “the premises at 11359 Happy Glade Avenue between the hours of 8 a.m. to 6 p.m.,” and direct the police to search for and seize “cash, betting slips, record books, and every other means used in connection with placing bets on horse races and other sporting events.”
The “knock-notice” or “knock-and-announce” rule derives from the Fourth Amendment’s protection against unreasonable searches and seizures. In general, even if officers have a search or arrest warrant that justifies their entering a home, they must announce themselves and their purpose before intruding.