Kurt Volker, the U.S. special representative to Ukraine, resigned Friday amid fallout from a whistleblower complaint against President Donald Trump regarding a phone call in July with Ukraine’s president.

Volker was one of five State Department officials that House Democrats said Friday they want to depose as part of an impeachment inquiry of Trump. The whistleblower complaint focuses on Trump’s July 25 phone call with Volodymyr Zelensky, the Ukrainian leader.

Beginning earlier this year, Volker served as a liaison between officials in the incoming Zelensky administration and Rudy Giuliani, the Trump lawyer who pushed the Ukrainians to investigate Joe Biden and his son, Hunter. (RELATED: House Democrats Subpoena Mike Pompeo)

It is unclear what role Volker played in helping Giuliani dig up dirt on the Bidens. The Trump lawyer has accused the former vice president of pressuring Ukraine’s president in 2016 to fire a prosecutor who was planning to investigate a Ukrainian energy company linked to Hunter Biden.

Volker helped Giuliani set up meetings with Zelensky aides. But the whistleblower complaint also says that Volker and Gordon Sondland, the ambassador to the European Union, met with Giuliani in an effort to “contain the damage” to national security.

Volker and Sondland also met with Ukrainian officials to help them understand the “different messages” they were receiving from Giuliani and official diplomatic channels.

Volker, who served as ambassador to NATO under George W. Bush and Barack Obama, joined the Trump State Department in 2017. He was also executive director at the McCain Institute, founded by late Arizona Sen. John McCain.

The State Press, the university newspaper for Arizona State University, which is affiliated with the McCain Institute, first reported Volker’s resignation.

The career diplomat has never been considered a Trump insider. He was brought to Foggy Bottom by Rex Tillerson, who infamously butted heads with Trump while serving as secretary of state.

On Friday, Democrats on the House Intelligence, House Foreign Affairs, and House Oversight Committees requested a deposition with Volker and four other State Department officials identified in the whistleblower complaint. The Democrats also subpoenaed Secretary of State Mike Pompeo.

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