In the 1961 case of Mapp v. Ohio, by a 6-3 margin, the Supreme Court endorsed the principle that the Fourth Amendment applied to states as well as the federal government

The Fourth Amendment of the Constitution provides that “the right of the people to be secure in their persons, houses, papers and effects, against unreasonable searches and seizures, shall not be violated, and no warrants shall issue, but upon probable cause, supported by oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.”

That provision was adapted from English common law practice. But it leaves unanswered some important questions. What exactly is a “search and seizure”? And can evidence procured in an illegal search and seizure be used by the government to secure a criminal conviction?

The Supreme Court answered the second question in the 1914 case of Weeks v. United States. The defendant stood accused of using the mail to transport lottery tickets, in violation of federal law. At trial, the government used evidence uncovered in a search of his house for which a warrant was not obtained. The Court ruled that the evidence could not be used and that the conviction must be overturned.

The Weeks decision — and what was referred to as the exclusionary rule — applied only to federal criminal cases, and the FBI and other federal law enforcement agencies typically obtained search warrants before conducting searches, in order to be assured that evidence would be admissible. But at the time Weeks was decided, the Supreme Court held that the provisions of the Bill of Rights did not necessarily apply to state criminal cases.

Defendants did have the right to bring civil suits against state and local law enforcement officials for violation of privacy and recover money damages.

But such remedies were not often available or feasible to convicted criminals who tended to lack funds and credibility.

In the 1961 case of Mapp v. Ohio, the Supreme Court considered the issue at the state level. The defendant was convicted of violating gambling laws in Ohio, with evidence from a search conducted either without a warrant or without one that was produced at trial. On appeal, she argued that officers had no probable cause for the search. By a 6-3 margin, the Court overturned the conviction, and five justices explicitly endorsed the principle that the Fourth Amendment applies to states as well as the federal government.