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In addition, the Army has been investigating whether Mr. Flynn received money from the Russian government during a trip he took to Moscow in 2015, according to two defense officials. Such a payment might violate the Emoluments Clause of the Constitution, which prohibits former military officers from receiving money from a foreign government without consent from Congress. The defense officials said there was no record that Mr. Flynn, a retired three-star Army general, filed the required paperwork for the trip.
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https://www.nytimes.com/2017/02/13/us/politics/donald-trump-national-security-adviser-michael-flynn.html?action=Click&contentCollection=BreakingNews&contentID=64910606&pgtype=Homepage