MR. JUSTICE STEWART delivered the opinion of the Court.
The petitioner was convicted in the District Court for the Southern District of California under an eight-count indictment charging him with transmitting wagering information by telephone from Los Angeles to Miami and Boston, in violation of a federal statute. [Footnote 1] At trial, the Government was permitted, over the petitioner's objection, to introduce evidence of the petitioner's end of telephone conversations, overheard by FBI agents who had attached an electronic listening and recording device to the outside of the public telephone booth from which he had placed his calls ...
... What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection ...
The Government stresses the fact that the telephone booth from which the petitioner made his calls was constructed partly of glass, so that he was as visible after he entered it as he would have been if he had remained outside. But what he sought to exclude when he entered the booth was not the intruding eye -- it was the uninvited ear ... No less than an individual ... in a friend's apartment ... a person in a telephone booth may rely upon the protection of the Fourth Amendment. One who occupies it, shuts the door behind him, and pays the toll that permits him to place a call is surely entitled to assume that the words he utters into the mouthpiece will not be broadcast to the world ...
... the Government's position is that its agents acted in an entirely defensible manner: they did not begin their electronic surveillance until investigation of the petitioner'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 the petitioner's unlawful telephonic communications ...
Accepting this account of the Government's actions as accurate, it is clear that this surveillance was so narrowly circumscribed that a duly authorized magistrate, properly notified of the need for such investigation, specifically informed of the basis on which it was to proceed, and clearly apprised of the precise intrusion it would entail, could constitutionally have authorized, with appropriate safeguards, the very limited search and seizure that the Government asserts, in fact, took place. Only last Term we sustained the validity of such an authorization ...
... The government agents here ignored "the procedure of antecedent justification . . . that is central to the Fourth Amendment" ... Because the surveillance here failed to meet that condition, and because it led to the petitioner's conviction, the judgment must be reversed.
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