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In general, a state prosecutor cannot use evidence that police obtained through an unlawful search. This is because evidence obtained through an unlawful search is considered "fruit of the poisonous tree" and is considered tainted.

The exclusionary rule is a legal principle that prohibits the use of evidence in criminal proceedings that was obtained illegally. The purpose of the exclusionary rule is to deter police from engaging in unlawful searches and seizures and to protect the Fourth Amendment rights of individuals.

If the police obtain evidence through an unlawful search, the defendant can file a motion to suppress the evidence, arguing that it should not be used against them at trial. If the court grants the motion, the evidence will be excluded from the trial.

However, there are some exceptions to the exclusionary rule. For example, if the police obtained the evidence through good faith reliance on a valid search warrant that was later found to be invalid, the evidence may still be admissible.

This case answers the above question: Mapp v. Ohio, 367 U.S. 643 (1961).

This case asked the Court to revisit the issue of whether evidence obtained as a result of an illegal search and seizure, in violation of the Fourth Amendment, can be used against a defendant in a state—rather than federal—prosecution. Id. at 660. The Court had previously held, in Wolf v. Colorado, 338 U.S. 25 (1949), that the Fourth Amendment’s prohibition against unreasonable searches and seizures was made applicable against the state police via the Fourteenth Amendment—a process known as incorporation. Id. at 650-51. However, the Wolf Court had determined that the exclusionary rule, which was pronounced in Weeks v. United States, 323 U.S. 383 (1914), and holds “the Fourth Amendment bars the use of evidence secured through an illegal search and seizure” in any federal prosecution, did not apply to the States through incorporation. Id. at 651.

The petitioner in this case had been subjected to a warrantless search by three Cleveland police officers, which uncovered “lewd and lascivious books, pictures and photographs.” Id. at 643. These illegally seized items were used to convict petitioner under an Ohio statute prohibiting possessing such materials. Id. The Ohio Supreme Court found no error in using these items because Ohio was not bound by the exclusionary rule pursuant to Wolf v. Colorado. Id. at 645. Instead, Ohio had fashioned its own version of the exclusionary rule, which only prohibited using evidence taken “from the defendant’s person by the use of brutal or offensive physical force.” Id.

The Court revisited the reasoning behind Wolf, which had declined to incorporate the exclusionary rule based upon “the experience of States which deem the incidence of such conduct by the police too slight to call for a deterrent remedy” and fear of inappropriately overriding a state’s rule of evidence. Id. at 651. However, this power and freedom left to the States had led to mass confusion. Id. at 653. For example, the Court had disallowed the use of evidence illegally obtained by a state official in a federal prosecution. Id. at 653. Nor could a state prosecution use evidence illegally obtained by a federal official. Id. The Court found these holdings had whittled away the holding of Wolf and counseled against its reasoning. Id. Finally, the Court felt sufficient time had passed since the pronouncement of incorporation of the Fourth Amendment to justify the enforcement of the exclusionary rule upon the States without surprise. Id. at 654-55.

The Court reasoned that “denial of [the] most important constitutional privilege, namely the exclusion of the evidence which an accused had been forced to give by reason of unlawful seizure” is just as heinous when conducted by a state official as a federal official. Id. at 656. This reasoning is identical to that used to support the original adoption of the exclusionary rule. Id. at 659. In concluding, the Court noted “the ignoble shortcut to conviction left open to the Stated tends to destroy the entire system of constitutional restraints on which the liberties of the people rest.” Id. at 660. The exclusionary rule was held to apply to all illegally obtained evidence in all prosecution. Id.