For example, if the NSA, while unconstitutionally listening to the conversations of Americans hoping to hear about plots to harm other Americans (it has revealed no such plots from among the trillions of private conversations it has monitored since 2005), comes across evidence of a bank robbery, the NSA will pass that evidence on to the Department of Justice (DOJ). The NSA routinely does this notwithstanding representations to the FISA court that authorizes its spying that it is not in the business of gathering evidence in criminal cases.
So, what happens when the spying uncovers ordinary criminal behavior unrelated to national security? In order to keep its hands clean, so to speak, the NSA sends that evidence to the DOJ, whose lawyers and agents in cahoots with the NSA then concoct an explanation as to how the DOJ came upon the evidence. Of course, that explanation curiously and carefully omits the mention of domestic spying. DOJ lawyers know that if the beginning of the process of obtaining evidence is found to be unconstitutional, then the evidence itself can be useless in court.
This is what lawyers and judges call the “fruit of the poisonous tree.” Were this not so—that is, if the government could spread any net as broad and as wide as it wished and use whatever the net caught as evidence in criminal prosecutions—then the Fourth Amendment’s search warrant requirement would be meaningless because it would not protect the right to privacy as its authors intended.
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