Congress continues closed-door depositions this week regarding the growing impeachment inquiry into President Donald Trump, and on Monday Fiona Hill, a former top national security adviser on Russia who left the administration just before the president's July phone call with the president of Ukraine, met with lawmakers behind closed doors for roughly 10 hours.
Democrats have accused Trump of abusing his office and potentially violating campaign finance laws. Despite admitting he wanted Democratic rival and former Vice President Joe Biden, and Biden's son, Hunter, investigated by Ukraine for alleged corruption, Trump has insisted he did nothing wrong.
Hill’s lawyer, Lee Wolosky, tweeted Monday morning that his client was subpoenaed by Democrats ahead of her testimony. Sources told ABC News that Hill was appearing as a voluntary witness and that the White House did not attempt to block her testimony.
An official working on the impeachment inquiry told ABC News that the committee subpoenaed Hill "in light of attempts by the White House and the Administration to direct witnesses not to cooperate with the House's impeachment inquiry and efforts by the White House to limit any testimony that does occur."
"As is required of her, Dr. Hill is now complying with the subpoena and answering questions from both Democratic and Republican Members and staff," the official said.
"She was a remarkably thorough and authoritative witness who had truly amazing powers of recall of particular events and particular meetings," Rep. Jamie Raskin, D-Md., told reporters late Monday.
A copy of the request for documents and testimony lawmakers issued to Hill last week, obtained by ABC News, revealed a broad spectrum of issues the Democrat-led committees hoped she could shed light on. The request included information about the efforts by any current or former Trump administration officials -- as well as the president's personal lawyer, Rudy Giuliani, and newly indicted Soviet-born Florida-based businessmen Lev Parnas and Igor Furman -- to investigate matters related to Burisma Holdings, former Trump campaign chairman Paul Manafort, Hunter and Joe Biden, the Democratic National Committee, Hillary Clinton and former U.S. Ambassador to Ukraine Marie "Masha" Yovanovitch.
"The July 25 phone call "wasn't a one-off," Raskin told reporters. "It was the culmination of a continuing campaign by Rudy Giuliani and his henchmen, as well as other public actors ... to undermine ambassador Yovanovitch."
Hill praised Yovanovitch behind closed doors with lawmakers, according to a source familiar with her testimony, and opposed her recall last spring.
GOP Reps. Lee Zeldin of New York and Jim Jordan of Ohio did not comment on the deposition after it was over, but criticized Democrats for running what they consider to be an unfair investigation against Trump.
(MORE: Giuliani's relationship with arrested men subject of criminal investigation: Sources)Before her most recent work in the White House, Hill served under both President George W. Bush and President Barack Obama as a career national intelligence officer for Russia and Eurasia on the National Intelligence Council.
An accomplished scholar and author on modern Russia, and a sharp critic of President Vladimir Putin, Hill initially was recruited to the post by Trump's first national security adviser, Michael Flynn, and his then-deputy national security adviser, K.T. McFarland, according to a source. Hill officially joined the national security team under the leadership of Flynn's successor, former national security adviser H.R. McMaster, in April 2017.
Hill, who holds a master's in Soviet studies and a doctorate in history from Harvard University, is described by former colleagues as the ultimate expert on all things related to Russian foreign policy, with a great wealth of institutional knowledge of Putin's domestic and international strategic goals. She is widely praised for her work on the NSC, and multiple individuals close to her have said they're amazed she lasted so long in the Trump administration.
(MORE: Trump claims on Ukraine call may stem from foreign official's year-old request)"Hill would be able to describe to the committees discrepancies, observations and recommendations emanated by inter-agency discussions organized by the NSC, and if and how those recommendations were acted upon by the president," said ABC News Contributor John Cohen, a former DHS acting undersecretary who's worked as both a congressional and federal investigator.
"She would have access -- and would have been involved in -- senior discussions on Russia and other issues, with visibility in internal discussions within the White House on Russia and Ukraine," Cohen added.
Hill is presently on leave from The Brookings Institute in Washington, where she directed the Center on the United States and Europe from 2009 to 2017.
Reached by ABC News, Wolosky declined to comment ahead of Hill's testimony. The Brookings Institute also declined to comment.
Deputy Assistant Secretary of State George Kent is scheduled to appear Tuesday morning for a deposition. Tuesday is also the deadline for the vice president's office to produce documents to the committee voluntarily, and for the Pentagon, Office of Management and Budget, and Rudy Giuliani to do so under subpoena(s).
ABC News' Benjamin Siegel, Katherine Faulders, Sarah Kolinovsky, Mariam Khan and Allison Pecorin contributed to this report.