Fiona Hill, a former top National Security Council expert on Russia, praised ousted U.S. Ambassador to Ukraine Marie Yovanovitch during testimony on Monday before the House Intelligence Committee.

Fox News has learned that Hill, who is testifying before the committee as part of the House's impeachment inquiry into President Trump, had high praise for Yovanovitch during the closed-door session.

Yovanovitch, who herself testified before congressional lawmakers last Friday as part of the impeachment probe, was recalled from her ambassadorship in May by Trump, around the same time that Rudy Giuliani — the onetime New York mayor is Trump's personal attorney and has no official role in the U.S. government — pressed Ukrainian officials to investigate corruption allegations against Democrat Joe Biden and his son Hunter, who was involved with the board of a gas company there.

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The Trump administration and some Republican lawmakers have argued that Yovanovitch’s removal from the job had nothing to do with Trump’s moves to have the Bidens investigated. But The Wall Street Journal reported earlier this month that she was recalled from Kiev after months of complaints by Giuliani that she'd been “undermining him abroad and obstructing efforts to persuade” Ukrainian leaders to investigate Biden.

Trump, in a July 25 phone call with his counterpart in Ukraine, told Ukrainian President Volodymyr Zelenskiy that Yovanovitch was "bad news," according to a partial transcript released by the White House. Neither Giuliani nor Trump has publicly specified their objections to her.

Yovanovitch said in her statement Friday that she was abruptly told this spring to depart Ukraine "on the next plane." She left her post in May and was later told the president had lost confidence in her and had been pressuring State Department officials for many months to remove her, she said.

Democrats leading the investigation said they subpoenaed Yovanovitch on Friday morning after learning late Thursday that the State Department had directed her not to appear. Trump has forbidden all government employees to cooperate; Yovanovitch remains employed by the State Department and is doing a fellowship at Georgetown University.

The House committees investigating Trump have ramped up their efforts to bring in people familiar with the Ukraine actions by Trump and Giuliani. Following Hill’s testimony on Monday, Gordon Sondland, Trump's hand-picked ambassador to the European Union, is poised to appear on Capitol Hill on Thursday.

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This follows the revelation of a cache of text messages from top envoys that provide a vivid account of their work acting as intermediaries around the time Trump urged Zelenskiy to start investigations into a company linked to Hunter Biden.

Sondland is set to tell lawmakers that he did understand the administration was offering Zelenskiy a White House visit in exchange for a public statement committing to investigations Trump wanted, according to the person, who demanded anonymity to discuss remarks not yet given.

But Sondland will say he did not know the company being talked about for an investigation, Burisma, was tied to Hunter Biden. Sondland understood the discussions about combating corruption to be part of a much broader and publicized Trump administration push that was widely shared.

The Associated Press contributed to this report.