WASHINGTON (Reuters) - Donald Trump’s former Russia adviser testified for more than nine hours on Monday behind closed doors, the latest witness summoned in the impeachment inquiry against the U.S. president over his request that Ukraine investigate a political rival.

The Democrats’ rapidly progressing inquiry could prompt the House of Representatives to approve articles of impeachment - formal charges - leading to a trial in the Senate on whether to remove Trump from office.

Fiona Hill, former senior director for European and Russian Affairs on Trump’s National Security Council, recounted a July 10 meeting in Washington that she attended with senior Ukrainian and U.S. officials, including U.S. Ambassador to the European Union Gordon Sondland, according to a person familiar with her testimony.

Hill said Sondland raised the matter of investigations, which she and others took as a reference to a probe into Democratic presidential candidate Joe Biden and his son Hunter, who had served on the board of a Ukrainian gas company, said the person, who spoke on condition of anonymity.

Alarmed at what she heard, Hill said she left the meeting and was advised to see National Security Council lawyer John Eisenberg, the person said.

Hill said Sondland appeared to be coordinating with Mick Mulvaney, the acting White House chief of staff, the person said.

The meeting happened about two weeks before a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate unsubstantiated allegations against the Bidens, the call at the center of the investigations.

The White House did not immediately respond to a request for comment.

Democratic Representative Jamie Raskin told reporters Hill, who spoke to the House Intelligence Committee and two other panels, “was a remarkably thorough and authoritative witness” who recalled “particular events and particular meetings”.

As they have done in the past, Republican lawmakers who have shown little inclination to remove Trump complained that the hearings were closed to the public and that transcripts were not being released.

The pace of the House Democrats’ investigation quickened on Monday as they lined up additional witnesses to testify in closed sessions.

Michael McKinley has agreed to appear voluntarily for a closed transcribed interview on Wednesday, just days after he announced his resignation as a senior adviser to Secretary of State Mike Pompeo.

Also, a person familiar with the proceedings confirmed Deputy Assistant Secretary of Defense Laura Cooper would be interviewed by the committees on Friday.

Trump has denied wrongdoing in his dealings with Ukraine.

Adding to the almost dizzying number of developments, the Wall Street Journal reported federal investigators for at least the past couple of months have been questioning witnesses as they look into Rudy Giuliani’s business dealings in Ukraine. Giuliani has been acting as Trump’s personal lawyer.

Justice Department officials in Washington referred a request for comment to the U.S. District Court in the Southern District of New York, where a spokesman declined to comment.

Giuliani faces a Tuesday deadline to produce documents related to the Ukraine matter subpoenaed by the House Intelligence Committee. He has not said whether he will comply.

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The former New York mayor has defended his actions as proper in his role as Trump’s lawyer.

‘UNFOUNDED, FALSE CLAIMS’

The Trump administration’s removal in May of Marie Yovanovitch as U.S. ambassador to Ukraine may have figured in Hill’s testimony.

On Friday, Yovanovitch testified she had been ousted based on “unfounded and false claims” after coming under attack by Giuliani. Giuliani had been working to get Ukraine to investigate Biden, a former vice president and contender for the Democratic nomination to face Trump in the November 2020 presidential election.

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Democrats have accused Trump of pressuring a vulnerable U.S. ally to dig up dirt on a domestic rival after withholding $391 million in U.S. security aid. Zelenskiy agreed to investigate and Trump eventually allowed the aid.

The British-born Hill, who left her White House job shortly before the July 25 call but remained on the payroll into August, had advocated a tough U.S. approach on Moscow even as Trump was more accommodating toward Russian President Vladimir Putin.

The White House demanded Hill limit her testimony to the contents of the July 25 telephone call and warned she could not discuss internal deliberations or diplomatic communications about the conversation, according to a letter dated Monday from a White House lawyer to Hill’s attorney that was seen by Reuters.

In the letter to attorney Lee Wolosky, White House Deputy Counsel Michael Purpura cited the doctrine of executive privilege, which is intended to protect communications between the president and close advisers to ensure candid, private advice.

Purpura acknowledged that because Trump released the summary of his call with Zelenskiy, it was not covered by executive privilege. But, he wrote, Trump had not waived executive privilege applying to internal deliberations surrounding the call or to other classified materials.

Representatives for Wolosky and the White House did not immediately respond to a requests for comment on the letter.

On Thursday, committees are scheduled to receive testimony from Sondland, the U.S. ambassador to the EU.

Sondland, a Trump backer and not a career diplomat, participated in a text message exchange with Bill Taylor, the top U.S. diplomat in Ukraine. Democrats say it reveals concern among aides that Trump’s pressure on Ukraine to investigate Biden was improper.

Sondland is expected to be asked why he relayed from Trump to other diplomats that the president said no “quid pro quos” connecting the Biden investigation with the U.S. aid. Quid pro quo is a Latin term meaning a favor for a favor.

WHISTLEBLOWER TESTIMONY

As talks continue over whether the whistleblower from within the U.S. intelligence community who prompted the inquiry will testify, Trump weighed in on Twitter, demanding the person testify and the individual’s identity be revealed.

Negotiations between representatives for the whistleblower and congressional committees were deadlocked over Senate Intelligence Committee requests that the individual provide testimony in person, while the House committees were willing to exchange questions and answers in writing, according to two people familiar with the negotiations.

The House panels also are scheduled this week to hear from Deputy Assistant Secretary of State George Kent and State Department Counselor Ulrich Brechbuhl, a top aide to Pompeo.

Lawmakers also may debate whether to seek to compel testimony from Giuliani. On Thursday, two Giuliani associates - Ukraine-born Lev Parnas and Belarus-born Igor Fruman - who helped him with his efforts to investigate the Bidens - were charged with scheming to violate U.S. campaign finance laws.