“ The agent therefore had access to a great mass of illegally obtained information unmistakably suggesting a line of further inquiry, as well as to the isolated fact that a letter-of unknown contents-to the scrap metal company had been delivered many months before to an address associated with petitioner. ”

“ It presents a fundamental question of law: The Government contends and the courts below found that the federal agents obtained incriminating evidence relating to unreported income from a scrap metal company as a result of assertedly nontainted leads gleaned from the mail cover. ”

“ FBI and IRS agents also kept the secretary's office-apartment under observation through a peep-hole in the door of a room across the hall and at the request of IRS officials, the Post Office maintained a mail cover of the petitioner, recording the addressee and sender of all mail delivered to certain addresses believed to be connected with petitioner. ”