Washington, DC – Democrats pursuing an impeachment inquiry of Donald Trump questioned the United States’s president’s former special envoy to Ukraine for several hours behind closed doors on Thursday.

Kurt Volker, a former US ambassador to NATO, resigned last week hours after being called by Democrats to be interviewed as part of the inquiry. Volker, named in a whistle-blower complaint about the president’s dealings with Ukraine, introduced Trump lawyer Rudy Giuliani to top Ukrainian officials.

The complaint alleges that Trump used “the power of his office to solicit interference from a foreign country in the 2020 US election”. Trump denies any wrongdoing.

At the centre of the complaint is a phone call in July between Trump and Ukrainian President Volodymyr Zelensky in which the US leader asked for help with investigating Joe Biden, the former US vice president and a 2020 Democratic hopeful and his son.

The complaint said Volker tried to “contain the damage” from efforts of Giuliani to get Ukraine to look into Biden.

“Guiliani, he’s running a shadow shakedown,” Democrat Eric Swalwell told reporters after Volker’s testimony. He said Democrats would release text messages and other documents related to Volker’s testimony.

Volker told members of the House that Giuliani was warned Ukrainian claims of alleged corruption by Biden and his son Hunter were not credible, the Washington Post newspaper reported, citing to unnamed people familiar with his testimony.

“We have ample evidence now that there was a requirement that President Zelensky investigate the 2016 election and the Bidens if he wanted to get a meeting, that that was in the conversation throughout the time that President Zelensky came into office,” Swalwell said.

Republicans who sat in on the interview said Volker’s testimony did not support the allegation that Trump withheld $400m in military aid from Ukraine to pressure Zelensky to investigate Biden.

“The argument that President Trump was trying to get President Zelensky to manufacture dirt on the Bidens, Ambassador Volker’s testimony blew a massive hole in that argument,” Republican Lee Zeldin told reporters.

Text messages

As Volker was testifying, Giuliani tweeted copies of text exchanges with Volker and Gordon Sondland, the US ambassador to the European Union. Sondland and Volker met Zelensky the day after the phone call with Trump, according to the whistle-blower complaint.

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“Rudy, just talked to Volker. Gave me a Zelensky update. I have been in touch directly w Zelensky as well. You able to meet in NY or DC Mon morning? I have some EU issues to discuss,” Sondland’s text message read.

Giuliani characterised the texts as evidence he was working with the full knowledge of US diplomats.

Volker provided 60 pages of documents, mostly text messages, prior to his interview with the House committee, according to the New York Times newspaper. Zeldin said the text messages should be publicly released.

Volker’s appearance before House members came against another explosive day for Trump, who publicly said China should also investigate Biden.

Trump made the comments to reporters on Thursday morning. On Wednesday, Trump railed against reporters and ramped up his rhetoric and tweets against the impeachment inquiry, which he describes as a “hoax” and “witch-hunt garbage”.

There has been no evidence of wrongdoing by Joe Biden or his son.