A police officer made a routine traffic stop, lacking any probable cause, and asked for permission to search the car. The brother of the car's owner gave consent. Upon searching the vehicle, the officer discovered three stolen checks, which were later linked to Robert Bustamonte (defendant), one of the six passengers riding in the car. Over Bustamonte’s objections, the state trial court allowed the evidence of the checks to be admitted at trial, and Bustamonte was convicted of theft. The state appellate court upheld the conviction. Bustamonte petitioned the federal district court for a writ of habeas corpus, but the district court denied Bustamonte's petition. The federal appellate court reversed and set aside the district court's order, holding that in order to prove voluntariness, the prosecution had to establish that the person giving consent knew he had the right to withhold consent. The United States Supreme Court granted certiorari.