A search warrant is a legal document that is signed by a judge. A search warrant gives the police permission to conduct a search in a specific place for specific items. In order to obtain a search warrant, the police must establish probable cause based on their own observations or based on information received from a reliable informant. In most cases, if the police conduct a search without a warrant, the search is presumed to be unreasonable. The police search generally cannot extend beyond the scope of the search warrant with regard to either the locations searched, or the items for which the police are searching.