The Warren Court ruled 6–3 in Mapp v. Ohio (1961) that the Fourth Amendment–based exclusionary rule—which holds that evidence illegally seized by law enforcement officials must be excluded from trial—applies to the states through the Fourteenth Amendment Due Process Clause. In 1949, the Court had ruled in Wolf v. Colorado that “in a prosecution in a State court for a State crime the Fourteenth Amendment does not forbid the admission of evidence obtained by an unreasonable search and seizure.” The Court overruled that aspect of its Wolf decision twelve years later in Mapp.

Justice Tom C. Clark, a former prosecutor, wrote: “We hold that all evidence obtained by searches and seizures in violation of the Constitution is, by that same authority, inadmissible in a state court.”