Former special envoy Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE is one of the first Trump administration officials testifying as part of the House’s impeachment inquiry.

Volker, who resigned from his position a week ago, is being deposed Thursday by three House panels behind closed doors.

The former envoy to Ukraine is seen as a key witness as House Democrats investigate communications between Kiev and President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE and his administration as part of the impeachment inquiry.

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Volker is a longtime diplomat

Volker was appointed to the unpaid, part-time envoy position in July 2017 and resigned Friday — a day after the release of a whistleblower report that said Trump pressed Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son, Hunter Biden; and that White House officials sought to cover it up.

Volker began his career as a CIA officer before working as a foreign service officer for the State Department during the Reagan administration. He was appointed as U.S. ambassador to NATO in 2008 by President George W. Bush.

A key player in whistleblower complaint

The whistleblower complaint raises “concerns” about how officials responded to Trump’s July 25 call to Zelensky, during which he appeared to ask a foreign leader to dig for dirt on a political rival.

The complaint alleges that officials aware of the call were worried that Trump was using his position for personal gain.

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Volker and Gordon Sondland, the U.S. ambassador to the European Union, allegedly traveled to Kiev just a day after the call to provide Zelensky and other Ukrainian political figures with advice on how to “navigate” Trump’s demands.

“Based on multiple readouts of the meetings recounted to me by various U.S. officials, Ambassadors Volker and Sondland reportedly provided advice to the Ukrainian leadership about how to ‘navigate’ the demands that the President had made of Mr. Zelensky,” the whistleblower wrote in the formal complaint.

The whistleblower also noted that Volker and Sondland spoke with Trump’s personal attorney Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE as part of an effort to “contain the damage” to national security.

“Officials also told me … that Ambassadors Volker and Sondland during this time period met with members of the new Ukrainian administration and, in addition to discussing policy matters, sought to help Ukrainian leaders understand and respond to the differing messages they were receiving from official U.S. channels on the one hand, and from Mr. Giuliani on the other,” the complaint reads.

Giuliani said Volker set up his meetings with Ukrainian officials

Giuliani last month posted a July 19 text message allegedly from Volker that connected Trump’s personal attorney to Andriy Yermak, a top adviser to Zelensky.

Giuliani then went on Fox News and displayed additional text messages from Volker directing him to meet with Ukrainian officials.

“He should step forward and explain what he did,” Giuliani told Fox News host Laura Ingraham Laura Anne IngrahamTrump assails Black Lives Matter in appeal to Black voters Ex-Pence aide: Trump spent 45 minutes of task force meeting 'going off on Tucker Carlson' instead of talking coronavirus Sean Hannity and Lou Dobbs to be deposed in Seth Rich lawsuit: report MORE. “The whistleblower falsely alleges that I was operating on my own. Well, I wasn’t operating on my own.”

The whistleblower complaint alleges Giuliani traveled to Madrid to meet with Yermak “on or about 2 August” … “as a ‘direct follow-up’ to the President’s call with Mr. Zelensky about the ‘cases’ they had discussed.”

Sen. Chris Murphy Christopher (Chris) Scott MurphyDemocratic senator calls for 'more flexible' medical supply chain to counter pandemics The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon GOP chairman to release interim report on Biden probe 'in about a week' MORE (D-Conn.) called Giuliani’s text messages proof of "highly illegal coordination between the Trump campaign and the State Department."

This isn’t real, is it? Giuliani didn’t just voluntarily expose a highly illegal coordination between the Trump campaign and the State Department? https://t.co/iESL7UgxSx — Chris Murphy (@ChrisMurphyCT) September 26, 2019

Giuliani continued to hammer Volker over his role in arranging meetings during Volker’s testimony on the Hill.

Pompeo tried to block Volker’s testimony

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As part of the formal impeachment inquiry, chairmen from multiple House committees subpoenaed Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE for documents related to the Trump administration’s dealings with Ukraine. They also called for depositions with five current and former department officials, including Volker.

Pompeo on Tuesday dismissed Democrats’ demands, arguing that that the requests amounted to an attempt to “intimidate” and “bully” the State Department. He also claimed that the House’s request for testimony raised “significant legal and procedural concerns.”

Pompeo on Wednesday admitted to being on the July 25 phone call in which Trump pressured Zelensky to investigate the Biden family. Asked during a press conference whether he heard anything concerning on the call, Pompeo defended the administration’s policy toward Ukraine.

“I know precisely what the American policy is with respect to Ukraine. It's been remarkably consistent, and we will continue to try to drive those set of outcomes,” Pompeo said.

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“It's what our team, including Ambassador Volker, were focused on was taking down the threat that Russia poses there in Ukraine. It was about helping the Ukrainians to get graft out and corruption outside of their government and to help now this new government in the Ukraine build a successful thriving economy. It's what the State Department officials that I've had the privilege to lead have been engaged in. And it's what we will continue to do. Even while all this noise is going on.”

Significant witness in impeachment probe

Volker is not believed to have been on the July 25 call with Trump and Zelensky. However, his insider knowledge of the relationship between the U.S. and Ukraine makes him a valuable witness in the House probe.

Prior to his testimony, Volker reportedly turned over a number of documents concerning his conversations with Giuliani and other officials. He is likely to be asked about that communication, as well as the anonymous whistleblower’s allegations about the intentions behind his interaction with Ukrainian officials.

Lawmakers may also want to hear an explanation for Volker’s abrupt resignation from his position as special envoy.