In its pursuit of establishing surveillance on the Trump campaign, the FBI turned its attention to Trump campaign adviser Carter Page in the spring of 2016, culminating in the issuance of a FISA warrant—which allows for some of the most intrusive spying methods on an American citizen.
As part of this process, the FBI relied extensively on the flawed Steele dossier, leading an FBI legal counsel to note that this was “essentially a single source FISA.”
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Summary via R3publicans