Updated 11:20 a.m. ET

As part of the congressional impeachment inquiry into President Trump, Marie Yovanovitch, the former U.S. ambassador to Ukraine who was recalled from her post by Trump, is testifying Friday before three House committees in a closed-door deposition.

The career diplomat was recalled from her post in May after criticism from some Republicans, including Donald Trump Jr., the president's eldest son, that she was not supportive of the administration's policies. She was appointed by President Barack Obama in 2016 and confirmed without any controversy by the GOP-led Senate.

She has emerged as a key fact witness in the House Democrats' investigation following a whistleblower complaint about Trump's telephone call with Ukrainian President Volodymyr Zelenskiy on July 25 that triggered the inquiry.

Investigators are interested in learning details about Yovanovitch's interactions with the president's personal lawyer Rudy Giuliani and others seeking a Ukrainian investigation into former Vice President Joe Biden and his son Hunter's business interests in one of the country's largest gas companies.

Trump referred to Yovanovitch as "bad news" to Zelenskiy in the White House account of the conversation released last month.

Earlier this week, the State Department blocked the U.S. ambassador to the European Union, Gordon Sondland, from testifying. After Congress issued a subpoena, Sondland said he would defy the State Department and testify Thursday.

An attorney for Sondland said in a statement that he has "at all times acted with integrity and in the interests of the United States. He has no agenda apart from answering the Committees' questions fully and truthfully."

The statement also noted that Sondland will not release documents that Congress is requesting, however, and that it's up to the State Department to do so.

"By federal law and regulation, the State Department has sole authority to produce such documents," per the statement, "and Ambassador Sondland hopes the materials will be shared with the Committees in advance of his Thursday testimony."

Sondland flew from Brussels to Washington to voluntarily answer questions about text messages between him and the senior diplomat at the U.S. Embassy in Ukraine, Bill Taylor, about whether military assistance was being conditioned on investigations related to political campaigns.

Earlier this week, White House counsel Pat Cipollone sent a letter to House Speaker Nancy Pelosi, D-Calif., and the chairs of the three committees saying the administration would not cooperate with document or testimony requests, arguing that the inquiry "lacks any legitimate constitutional foundation" and is simply an effort to "nullify the outcome of the democratic process" by reversing the results of the 2016 election and influence the upcoming 2020 election.

The whistleblower complaint released Sept. 26 says that Yovanovitch had criticized a former Ukrainian prosecutor's "poor record on fighting corruption." The same prosecutor, Yuriy Lutsenko, spread rumors about the Biden family and about Ukraine's "purported involvement in the 2016 U.S. election," as the whistleblower notes.

Lutsenko accused Yovanovitch of obstructing Ukrainian investigations and providing the U.S. with "evidence" of wrongdoing.

The whistleblower added that Lutsenko "later walked back" many of his allegations.

Despite that, the whistleblower cites reports that Giuliani met with Lutsenko twice, once in New York and once in Warsaw, Poland, earlier this year. Trump himself, as the whistleblower also notes, called Lutsenko's allegations "big" and "incredible" on Fox News and said "that the attorney general 'would want to see this.' "