In the case of McCray v. Illinois in 1966, the U.S Supreme Court upheld a conviction based on evidence obtained through an anonymous tip to police officers. The defendant, Robert McCray, was arrested and charged with possession of narcotics after police received information from an undisclosed informant that McCray would be carrying drugs at a specific location. At trial, McCray requested the identity of the informant but both the trial court and later appellate courts denied this request citing it could endanger future investigations or risk harm to informants. On appeal to the Supreme Court, it was argued that denying access to this information violated his constitutional rights under due process clause as well as confrontation clause which guarantees defendants right to confront their accusers in court. However, by a vote of 5-4 majority decision led by Justice Clark held that there is no absolute constitutional right for defendants to know identities of confidential informants if government shows reasonable cause for keeping them secret.

In the dissenting opinion for McCray v. Illinois, Justice William O. Douglas argued that the majority's decision undermined Fourth Amendment protections against unreasonable searches and seizures by allowing police officers to make arrests based on information from anonymous informants without requiring them to disclose the identity of those informants in court. He contended that this practice could lead to abuses of power and false arrests, as it would be difficult for defendants to challenge the credibility of unknown sources. Furthermore, he believed that such a rule was inconsistent with previous Supreme Court precedents which emphasized transparency in criminal proceedings and required law enforcement officials to provide specific reasons for their actions during an arrest or search.