Democrats have zeroed in on the State Department in the opening phase of their impeachment investigation. The Intelligence, Oversight and Foreign Affairs committees have already interviewed Kurt Volker, a former special envoy to Ukraine who provided the text messages. At least two other witnesses are set for depositions this week: Gordon Sondland, the U.S. ambassador to the European Union, and Marie Yovanovitch, who was abruptly ousted as the U.S. ambassador to Ukraine in May.