In the 1959 case of Abel v. United States, the U.S. Supreme Court ruled that evidence obtained by federal agents during a warrantless search could be used in court because it was not connected to an illegal arrest. The defendant, Rudolf Ivanovich Abel, who had been living under various aliases in New York City, was suspected of being a Soviet spy. Federal agents arrested him without a warrant and searched his apartment and hotel room where they found incriminating items such as coded messages and radio transmitting equipment which were later used as evidence against him at trial. Abel argued that this violated his Fourth Amendment rights against unreasonable searches and seizures but the Supreme Court disagreed stating that since there was probable cause for his arrest due to immigration violations (overstaying visa), any subsequent search did not violate his constitutional rights.

In the dissenting opinion for Abel v. United States, Justice Brennan argued that the majority's decision to uphold evidence obtained through a warrantless search was fundamentally flawed and violated Fourth Amendment protections against unreasonable searches and seizures. He contended that even if Abel was an illegal alien or unregistered foreign agent, he still had constitutional rights which were infringed upon by this action. Furthermore, Brennan criticized the government's use of administrative warrants as a guise to conduct criminal investigations without probable cause - something he saw as undermining basic principles of justice and fairness in law enforcement practices. He also disagreed with the majority’s view on “fruit of poisonous tree” doctrine application in this case; according to him, all evidence derived from initial unlawful arrest should have been excluded at trial regardless of its relevance or reliability.