A Missouri police officer stopped Tyler McNeely (defendant) for possible drunk driving. McNeely failed field sobriety tests and declined to have his blood alcohol concentration (BAC) tested. The officer then arrested McNeely and drove him to a hospital. At the hospital, McNeely again refused to take a BAC test. Without obtaining a warrant, the officer ordered hospital staff to conduct an involuntary BAC test. The test showed that McNeely's BAC exceeded the legal limit. The State of Missouri (plaintiff) prosecuted McNeely for drunk driving. The trial judge acquitted McNeely because of the officer's failure to obtain a search warrant before ordering the involuntary BAC test. The Missouri Court of Appeals referred the state's appeal to the Missouri Supreme Court, which affirmed the trial judgment. The United States Supreme Court granted Missouri's petition for review.