A longtime U.S. diplomat who served as President Donald Trump’s special envoy for Ukraine will tell his story to congressional committee staff on Thursday as part of a Democratic-led impeachment probe of the Republican president.

Kurt Volker resigned as special representative for Ukraine negotiations on Sept. 27, the day after the public release of a whistleblower complaint that described him as trying to “contain the damage” from efforts by Trump’s lawyer, Rudy Giuliani, to press Ukraine to investigate Democrats.

Volker’s meeting with the committee comes after an emotional day in the impeachment inquiry on Wednesday, when Trump railed at journalists during a news conference, resorted to the use of an expletive on Twitter, and called the probe “a hoax and a fraud” while pledging to co-operate.

Democratic lawmakers said earlier on Wednesday they were prepared to subpoena White House records about Trump’s July 25 telephone call with Ukrainian President Volodymyr Zelensky.

Charges that Trump pressured Zelensky to investigate former vice-president Joe Biden, a leading rival in Trump’s bid for re-election in 2020, helped prompt House of Representatives Speaker Nancy Pelosi to announce a formal impeachment investigation last week.

Donald Trump lashed out today after Democrats gave notice that they would subpoena the White House for documents related to the phone call with Ukraine that sparked the impeachment investigation. 1:29

On Thursday, staff from the House’s foreign affairs, intelligence and oversight committees will conduct a transcribed interview with Volker. The 54-year-old, also a former U.S. ambassador to NATO, is appearing voluntarily.

The briefing will take place behind closed doors, the first of a series of events in the probe, none of them public, scheduled within the 10 days. More are expected.

Late on Wednesday, Texas Congressman Mike McCaul, the top Republican on the House foreign affairs committee, criticized the session with Volker, saying foreign affairs committee Republicans were not being given equal representation or the chance to question Volker.

He also expressed concern that Eliot Engel, the Democratic House foreign affairs chairman, had ceded oversight of U.S. foreign policy to the intelligence committee because that panel, he said, would lead the questioning of Volker.

An intelligence committee aide said the panel’s staff would conduct the interview because Pelosi had designated it as the lead committee in the impeachment inquiry.

Intelligence committee investigators have been working with the foreign affairs and oversight committees, “both of which will have equal majority and minority representation,” the aide said in an email.

On Friday, Intelligence Community Insp. General Michael Atkinson will testify at an intelligence committee hearing. Marie (Masha) Yovanovitch, the U.S. ambassador in Kyiv until Trump recalled her in May before her normal term was up, will meet with committee staff on Oct. 11.

Trump has denied wrongdoing, repeatedly saying his call with Zelensky was “perfect.”

Pelosi dismissed that characterization at a news conference on Wednesday, saying: “It’s not perfect – it’s wrong.”

State Dept. official’s documents called ‘propaganda’

Some of Trump’s Republican allies have joined him in seeking to discredit California Democrat Adam Schiff, chair of the House’s intelligence panel, after a New York Times report on Wednesday that the whistleblower had contacted the committee for advice on how to proceed.

Both Republicans and Democrats familiar with the process said, however, that Schiff’s panel had acted properly in advising the person to contact an inspector general and seek legal counsel.

Staff from eight Senate and House congressional committees, including Republicans and Democrats, also reacted with surprise on Wednesday after a briefing by the State Department’s inspector general, Steve Linick.

Maryland Democrat Jamie Raskin holds up a document to reporters that was submitted to the House on Wednesday. Raskin and others in the party were left perplexed by some of the information they received as part of a closed-door briefing with the State Department’s inspector general. (Susan Walsh/The Associated Press)

At the briefing, held behind closed doors but not classified, Linick handed out packets of documents that the only lawmaker who attended, Democratic Rep. Jamie Raskin of Maryland, described as “propaganda and disinformation.”

Committee staff members who attended the briefing said it concerned a package of documents delivered to Secretary of State Mike Pompeo that included allegations against Biden and Yovanovitch.

A photograph of one document, whose source was not disclosed, described a discredited theory promoted by Trump allies that Yovanovitch was installed in her post by billionaire George Soros, a Democratic donor frequently attacked by far-right activists.

The impeachment probe could lead to approval of articles of impeachment — or formal charges — against Trump in the House. A trial on whether to remove Trump from office would then be held in the Republican-controlled Senate. Republicans, however, have shown little appetite for ousting him.