Fruit of the poisonous tree is a legal metaphor used to describe evidence that is obtained illegally. The logic of the terminology is that if the source (the "tree") of the evidence or evidence itself is tainted, then anything gained (the "fruit") from it is tainted as well. United States The doctrine underlying the name was first described in Silverthorne Lumber Co. v. United States, 251 U.S. 385 (1920). The term's first use was by Justice Felix Frankfurter in Nardone v. United States (1939). Such evidence is not generally admissible in court. For example, suppose a police officer obtained a key to a train station locker in the process of conducting a search of a home that was unconstitutional on the grounds that it violated the Fourth Amendment. Any evidence of a crime that came from that locker would most likely be excluded under the "fruit of the poisonous tree" legal doctrine. The testimony of a witness who is discovered through illegal means would not necessarily be excluded, however, due to the "attenuation doctrine", which allows certain evidence or testimony to be admitted in court if the link between the illegal police conduct and the resulting evidence or testimony is sufficiently attenuated. For example, a witness who freely and voluntarily testifies is enough of an independent intervening factor to sufficiently "attenuate" the connection between the government's illegal discovery of the witness and the witness's voluntary testimony itself. (United States v. Ceccolini, 435 U.S. 268 (1978)) The "fruit of the poisonous tree" doctrine is an extension of the exclusionary rule, which, subject to some exceptions, prevents evidence obtained in violation of the Fourth Amendment from being admitted in a criminal trial. Like the exclusionary rule, the fruit of the poisonous tree doctrine is intended to deter police from using illegal means to obtain evidence. The doctrine is subject to four main exceptions. The tainted evidence is admissible if: it was discovered in part as a result of an independent, untainted source; or it would inevitably have been discovered despite the tainted source; or the chain of causation between the illegal action and the tainted evidence is too attenuated; or the search warrant was not found to be valid based on probable cause, but was executed by government agents in good faith (called the good-faith exception). Similar doctrines in Europe This doctrine was also used by the European Court of Human Rights