In the US the issue of warrants is determined under Title 18 of the US Code. The law has been restated and extended under Rule 41 of the Federal Rules of Criminal Procedure.
Under the Fifth Amendment to the United States Constitution, most searches by the police require a search warrant. In order to obtain one, an officer must first prove that probable cause exists, although this can be based on hearsay evidence and can even be obtained by oral testimony given over a telephone. Both property and persons can be seized under a search warrant. Under the Fourth Amendment searches must be reasonable and specific.