In McElroy v. United States (1981), the Supreme Court ruled that a defendant's Sixth Amendment right to confront witnesses was not violated when an undercover police officer testified about statements made by an informant who did not appear in court due to safety concerns. The case involved a drug trafficking operation where the informant had introduced the undercover officer to the defendants, leading to their arrest and conviction. The defense argued that they should have been allowed to cross-examine this key witness. However, the Court held that because there were other corroborating pieces of evidence supporting his testimony and because revealing his identity could put him at risk, it was permissible for him not testify in person.

In the dissenting opinion for McElroy v. United States, Justice Brennan argued that the majority's decision to uphold a warrantless search of an automobile was inconsistent with previous Supreme Court rulings on Fourth Amendment protections against unreasonable searches and seizures. He contended that there were no exigent circumstances justifying a warrantless search in this case, as police had ample opportunity to secure a warrant before searching the vehicle. Furthermore, he disagreed with the majority's assertion that automobiles are inherently less private than homes due to their mobility and visibility from public spaces. In his view, these factors do not diminish individuals' reasonable expectations of privacy in their vehicles or justify blanket exceptions to Fourth Amendment requirements for warrants based on probable cause.