In this file photo, Kurt Volker, US special envoy for Ukraine, speaks during a news conference in Kiev on July 27, 2019. SERGEI SUPINSKY/AFP/Getty Images

Kurt Volker, US special envoy to Ukraine, has resigned one day after the release of a whistleblower report alleging a coverup by the White House of a call between President Trump and the Ukrainian leader, three sources familiar with the matter confirmed to CNN.

The State Department has not returned messages seeking comment.

The State Press, the school paper of Arizona State University, first reported the development.

What we know about Volker: He has become a key player in the unfolding scandal surrounding the whistleblower complaint concerning Trump's July 25 phone call with Ukrainian President Volodymyr Zelensky, in which Trump pressured Ukraine to investigate former Vice President Joe Biden and his son, Hunter.

CNN has reported that Ukrainian President Volodymyr Zelensky had joked with Volker about Rudy Giuliani, Trump's personal attorney, in a meeting before the phone call. Volker later set up the meeting between Giuliani and a Zelensky adviser in an effort to get the Biden matter out of official talks. There is no evidence of wrongdoing by either Joe or Hunter Biden.