(CNN) - Former U.S. special envoy for Ukraine Kurt Volker, who has become a key player in House Democrats' impeachment inquiry into President Donald Trump, announced Monday he is stepping down as executor director of Arizona State University's McCain Institute.
Amid rumors that he was planning to step down, Volker told staff at the institute last week that he had no imminent plans to resign.
Volker said it was whatever is best for the McCain Institute, the source said.
Cindy McCain, the late Sen. John McCain's widow who also serves as chairwoman of the institute's board of trustees, on Monday thanked Volker in a statement "for his dedication and hard work in helping to build the McCain Institute into the results-driven, action-oriented institution that it is today."
The McCain Institute "implements programs and initiatives aimed at making a difference in people's lives across a range of critical areas: leadership development, human rights, rule of law, national security, counterterrorism and combating human trafficking," according to the statement announcing Volker's departure.