How Ukraine envoy's resignation could affect his possible Congressional testimony

Kurt Volker, the State Department's special envoy for Ukraine, resigned Friday amid a formal impeachment inquiry of President Trump and his communications with the Ukrainian government, including the country's president, Volodymyr Zelensky. Volker did not provide a public explanation for leaving his post, but a source familiar with his decision said Volker concluded he could not perform the job effectively as a result of the recent developments.One person familiar with the matter told NBC News t