In the United States, in order for police and other law enforcement personnel to search for evidence, they need a warrant. They must also have probable cause (a good reason to believe they will find evidence of a crime). A search warrant must be signed by a judge or magistrate. The officer must provide a sworn statement of fact called an affidavit. If the judge or magistrate believes the affidavit establishes probable cause, he or she approves the warrant. A warrant is usually for a specific period of time and spells out exactly what the officers may search for and seize.