The government argued that that its agents acted in an entirely defensible manner: They did not begin their electronic surveillance until investigation of Katz’s activities had established a strong probability that he was using the telephone in question to transmit gambling information to persons in other States, in violation of federal law. Moreover, the surveillance was limited, both in scope and in duration, to the specific purpose of establishing the contents of Katz’s unlawful telephonic communications. The agents confined their surveillance to the brief periods during which he used the telephone booth, and they took great care to overhear only the conversations of the petitioner himself.

The government further argued that because its agents relied upon the decisions in Olmstead and Goldman where eavesdropping without a physical trespass was not a search within the meaning of the fourth amendment, and because they did no more here than they might properly have done with prior judicial sanction, the Court should retroactively validate their conduct.

But the court once again, disagreed with the government.

The court reasoned that even if the governments assertions  are taken as true, the inescapable fact is that the restraint the agents exercised in conducting the search was imposed by the agents themselves, not by a judicial officer. They were not required, before commencing the search, to present their estimate of probable cause for detached scrutiny by a neutral magistrate. They were not compelled, during the conduct of the search itself, to observe precise limits established in advance by a specific court order. Nor were they directed, after the search had been completed, to notify the authorizing magistrate in detail of all that had been seized. In the absence of such safeguards, this Court has never sustained a search upon the sole ground that officers reasonably expected to find evidence of a particular crime and voluntarily confined their activities to the least intrusive means consistent with that end. Searches conducted without warrants have been held unlawful notwithstanding facts unquestionably showing probable cause, for the Constitution requires that the deliberate, impartial judgment of a judicial officer be interposed between the citizen and the police. Over and again this Court has emphasized that the mandate of the (Fourth) Amendment requires adherence to judicial processes,’ and that searches conducted outside the judicial process, without prior approval by judge or magistrate, are per se unreasonable under the Fourth Amendment —subject only to a few specifically established and well-delineated exceptions.

The court found that none the narrow exceptions applied in this case. The search was not conducted incident to an arrest, it was not conducted under hot pursuit, nor was it conducted with Katz consent.