The United States v. Crews case in 1979 revolved around the Fourth Amendment, which protects against unreasonable searches and seizures. The defendant, Theodore Crews, was arrested without probable cause for a robbery he did not commit but was later identified as the perpetrator of another crime while in police custody. His photograph taken at arrest led to his identification by the victim of an unrelated assault and robbery. He argued that this evidence should be suppressed because it stemmed from his unlawful arrest. However, the Supreme Court ruled unanimously that since the victim had independently recognized him both before and after seeing his photo (which she would have seen regardless due to her independent recollection), there wasn't a direct causal link between his illegal arrest and subsequent identification; hence it didn't violate "fruit of poisonous tree" doctrine under exclusionary rule - meaning illegally obtained evidence cannot be used in court.

In the dissenting opinion for United States v. Crews, Justice Brennan disagreed with the majority's decision to allow evidence obtained through an unlawful arrest to be used in court. He argued that this violated the Fourth Amendment rights of the defendant and undermined constitutional protections against illegal searches and seizures. According to Brennan, allowing such evidence would incentivize police misconduct by giving them a reason to make unlawful arrests in hopes of obtaining incriminating evidence. Furthermore, he contended that it was impossible for courts to determine whether or not identification testimony was independent from an illegal arrest because they are inherently intertwined; thus any attempt at separating them is artificial and misleading. Therefore, he believed all fruits of an illegal arrest should be excluded from trial as per exclusionary rule doctrine.