American diplomat 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, a key figure in the Ukraine controversy, arrived unexpectedly on Capitol Hill Wednesday, entering the closed-door space where House members have been interviewing witnesses as part of their impeachment inquiry into 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.

Volker, who formerly served as the U.S. special envoy to Ukraine, was one of the figures named in the whistleblower complaint and one of the first officials to testify before the House about Trump and 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's efforts to get Ukrainian President Volodymyr Zelensky to open a corruption investigation into 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, a top 2020 Democratic White House hopeful, and his son.

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Two sources familiar with the matter said he was in the Capitol to review his deposition transcript, a standard procedure.

In his opening statement earlier this month, which was obtained by news outlets, Volker defended his record, praised ousted U.S Ambassador to Ukraine Marie Yovanovitch and said he had no involvement in the Trump-Zelensky phone call on July 25.

Both Republicans and Democrats seized on Volker's testimony as evidence to support their claims.

Democrats said Volker validated previous claims from a government whistleblower that the president had sought to pressure his Ukrainian counterpart into investigating the Bidens while withholding military aid to the country.

Hours after Volker's marathon closed-door testimony on Oct. 3, Democrats on the three committees leading the impeachment investigation released selected text messages Volker had provided between himself, U.S. Ambassador to the European Union Gordon Sondland and Bill Taylor, who is currently the top diplomat in Ukraine.



During a July 19 exchange between the three, Volker wrote about Trump's upcoming call with Zelensky: "Most impt is for Zelensky to say that he will help investigation — and address any specific personnel issues — if they are any." Volker noted that he had met Rudy Giuliani, Trump's personal lawyer, for breakfast that morning.



Six days later, just before the call took place, Volker told Andrey Yermak, a top advisor to Zelensky: "Heard from White House — assuming President Z convinces trump he will investigate / 'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington."

Meanwhile, Republicans said his testimony supported their claims that there was no promise to withhold Ukraine aid in exchange for a Biden investigation.

"He said there was no quid pro quo whatsoever," said Rep. Jim Jordan James (Jim) Daniel JordanHouse panel pulls Powell into partisan battles over pandemic Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election House passes resolution condemning anti-Asian discrimination relating to coronavirus MORE (Ohio), the top Republican on the House Oversight and Government Reform Committee.

Volker testified just days after resigning from his special envoy position. Days later, he stepped down as the head of the McCain Institute.