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 that Volker's resignation will likely enable him to be much freer in what he can say about his time at his post if he is called at some point to testify before Congress.The whistleblower complaint that sparked the impeachment inquiry alleges that Volker went to Kiev to help guide Ukrainian officials on how to handle Trump's alleged demands that the government investigate former Vice President Joe Biden's son, Hunter. He also reportedly spoke with Trump's personal lawyer Rudy Giuliani in an attempt to "contain the damage" to U.S. national security.Giuliani has said Volker encouraged him to meet with Ukrainian officials regarding the Biden family. That indeed appears to be the case, but The New York Times reports Volker was acting at the request of the Ukrainians, who were reportedly concerned about how Giuliani's attempts to procure information about the Bidens and other Democrats might affect their relationship with the U.S. Read more at NBC News and The New York Times.