Mapp vs. Ohio established that evidence discovered in violation of the Fourth Amendment cannot be used in a state prosecution. Previously, such seizures were ruled inadmissible in federal cases, but the states interpreted the amendment as not applicable to state-level prosecution. The Supreme Court ruled that the Fourth Amendment rights were incorporated into the Fourteenth Amendment, which guarantees due process of law at both the state and federal levels.
On May 23, 1957, Cleveland police received a tip that the home of Dollree Mapp might contain evidence of illegal gambling as well as the suspect in a recent bombing case. When officers approached Mapp to search her home, she demanded a warrant. Officers returned with a blank piece of paper they insisted was a warrant, and broke into her home and began to search. After a struggle over the piece of paper, Mapp was handcuffed. The officers found no gambling equipment and no sign of their suspect, but they did discover pornographic materials and charged Mapp with their possession. She was later convicted.
Mapp appealed on the basis that her Fourth Amendment rights had been violated by the fake search warrant. The case worked its way to the Supreme Court, and the decision on June 19, 1961 was 6 to 3 in favor of Mapp.