Writing for the Court, Justice William Brennan held that law enforcement officers violate the Fourth and Fourteenth Amendments when they seize a suspect and transport them to the police station for interrogation without probable cause. Brennan first established that “there can be little doubt that petitioner was ‘seized’ in the Fourth Amendment sense when he was taken involuntarily to the police station.” Rejecting New York’s argument that reasonable suspicion was sufficient to justify Fantigrossi’s order to pick up Dunaway for questioning under a flexible balancing test, Brennan emphasized that the Fourth Amendment’s probable-cause requirement is an absolute standard. He explained that “for all but those narrowly defined intrusions, the requisite ‘balancing’ has been performed in centuries of precedent and is embodied in the principle that seizures are ‘reasonable’ only if supported by probable cause.”

Brennan distinguished Dunaway’s involuntary detention from the limited street encounters authorized under Terry v. Ohio (1968), noting that Terry created a very narrow exception to the probable-cause requirement for swift, on-the-spot actions like weapons frisks that fell “far short of the kind of intrusion associated with an arrest.” In contrast, Brennan found that the treatment of Dunaway in this case was “in important respects indistinguishable from a traditional arrest” because he “was taken from a neighbor’s home to a police car, transported to a police station, and placed in an interrogation room.” Brennan warned that extending the Terry exception to encompass such severe, involuntary detentions would have dangerous consequences, writing that “any ‘exception’ that could cover a seizure as intrusive as that in this case would threaten to swallow the general rule that Fourth Amendment seizures are ‘reasonable’ only if based on probable cause.” Brennan asserted that “detention for custodial interrogation—regardless of its label—intrudes so severely on interests protected by the Fourth Amendment as necessarily to trigger the traditional safeguards against illegal arrest.”

Turning to the admissibility of Dunaway’s confessions, Brennan rejected the argument that providing Dunaway with Miranda warnings made up for the illegal seizure. Citing Brown v. Illinois (1975), he explained that “the exclusionary rule, . . . when utilized to effectuate the Fourth Amendment, serves interests and policies that are distinct from those it serves under the Fifth.” Brennan further explained that while satisfying Miranda and proving that a confession is voluntary is a mandatory threshold requirement under the Fifth Amendment, Fourth Amendment analysis requires evaluating the causal connection between the illegal arrest and the confession. Brennan warned that “[i]f Miranda warnings, by themselves, were held to attenuate the taint of an unconstitutional arrest, regardless of how wanton and purposeful the Fourth Amendment violation, the effect of the exclusionary rule would be substantially diluted.” Under such a relaxed standard, Brennan argued that police would be encouraged to execute arrests without warrants or probable cause “for questioning or ‘investigation,’” knowing that any derived evidence could be made admissible at trial “by the simple expedient of giving Miranda warnings.”

Applying the multi-factor attenuation test from Brown, Brennan analyzed “the temporal proximity of the arrest and the confession, the presence of intervening circumstances, . . . and, particularly, the purpose and flagrancy of the official misconduct.” Brennan asserted that the Rochester detectives’ actions had a clear “quality of purposefulness” because they embarked on an “expedition for evidence” and admittedly seized Dunaway without probable cause “in the hope that something might turn up.” Furthermore, because Dunaway made his first incriminating statement within an hour after he reached the police station and “confessed without any intervening event of significance,” there was no break in the causal chain. Ultimately, Brennan concluded that allowing the admission of Dunaway’s statements under these circumstances would essentially permit “law enforcement officers to violate the Fourth Amendment with impunity, safe in the knowledge that they could wash their hands in the ‘procedural safeguards’ of the Fifth.”