John Bolton, then the national security adviser, listens as President Trump speaks during a Cabinet meeting in February. Bolton's opposition to a pressure campaign to get Ukraine to investigate conspiracy theories may pit him against his former boss.
William Taylor, the acting U.S. ambassador to Ukraine, arrives on Tuesday at a closed session before the House Intelligence, Foreign Affairs and Oversight committees at the U.S. Capitol.
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Former White House adviser on Russia Fiona Hill leaves the Capitol on Monday after testifying before lawmakers as part of the House impeachment inquiry into President Trump.
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Fiona Hill, a former White House adviser on Russia, arrives on Capitol Hill on Monday ahead of her testimony in connection with the House impeachment inquiry into President Trump.