The U.S. intelligence official who “unmasked” the names of multiple private citizens affiliated with Trump’s team is someone “very well known, very high up, very senior in the intelligence world,” a source told Fox News on Friday.

Fox News reports:

Intelligence and House sources with direct knowledge of the disclosure of classified names told Fox News that House Intelligence Committee Chairman Devin Nunes, R-Calif., now knows who is responsible — and that person is not in the FBI.

For a private citizen to be “unmasked,” or named, in an intelligence report is extremely rare. Typically, the American is a suspect in a crime, is in danger or has to be named to explain the context of the report.

“The main issue in this case, is not only the unmasking of these names of private citizens, but the spreading of these names for political purposes that have nothing to do with national security or an investigation into Russia’s interference in the U.S. election,” a congressional source close to the investigation told Fox News.

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The unmasking of Americans whose communications apparently were caught up in surveillance under the Obama administration is a key part of an investigation being led by Nunes, who has come under fire from Democrats for focusing on that aspect.