Illinois v. Gates

Police received a detailed anonymous letter accusing the Gateses (defendants) of running an illegal drug operation, including specifics about how they obtained marijuana and when their next transaction would occur; police independently corroborated several of the letter's details, particularly its predictions about the Gateses' future travel plans, with only a minor discrepancy uncovered, and obtained a search warrant that turned up drugs, weapons, and other contraband. The Illinois Supreme Court held the anonymous letter and corroborating affidavit failed to establish probable cause under the traditional two-pronged test, and the case reached the U.S. Supreme Court.