In the United States v. Abel case of 1984, the Supreme Court ruled that evidence obtained through a warrantless search could be used to impeach a witness's credibility in court. The defendant, Reino Hayhanen (aka Abel), was convicted for espionage activities against the U.S., and during his trial, he called upon another spy as a defense witness who testified under an alias name. However, FBI agents had previously conducted an illegal search on this defense witness and discovered his true identity which they revealed at trial to discredit him. The Supreme Court held that although such evidence might not have been admissible in making out the government’s case-in-chief due to Fourth Amendment protections against unreasonable searches and seizures; it could still be used for impeachment purposes because its probative value outweighed any potential prejudice towards Abel.

In the dissenting opinion for United States v. Abel, Justice Brennan disagreed with the majority's decision to allow evidence obtained through a warrantless search by FBI agents. He argued that this violated the Fourth Amendment rights of Abel, who was suspected of being an illegal alien and Soviet spy. Brennan contended that while it is permissible to use information from a reliable informant as probable cause for arrest or search warrants, using such information to justify a warrantless search goes against established legal principles. Furthermore, he criticized the majority’s reasoning that because Abel was lawfully arrested on immigration charges any subsequent searches were justified under "search incident to lawful arrest" doctrine; he believed this rationale could be used in future cases as an excuse for unlawful searches without proper justification or oversight.