WASHINGTON – A former National Security Council expert on Russia became on Monday the first former White House official to testify in the House's formal impeachment investigation of President Donald Trump.

Fiona Hill, who had been the National Security Council's senior director for Europe and Russia, spent more than 10 hours fielding questions from lawmakers on three House committees behind closed doors.

She did not make any comments or answer questions from reporters, though lawmakers signaled her deposition was one of several helping House Democrats piece together the full picture of Trump's conduct with Ukraine — the central aspect of their quick moving impeachment inquiry.

However, The New York Times and NBC News reported Monday that Hill told Congress that former Trump national security adviser John Bolton had concerns over the push to solicit help from Ukraine. Citing "people familiar with the testimony," the Times reported that at one point Bolton urged Hill to tell White House lawyers about the effort by U.S. Ambassador to the European Union Gordon Sondland, White House chief of staff Mick Mulvaney and Rudy Giuliani, the president's personal lawyer.

"I am not part of whatever drug deal Sondland and Mulvaney are cooking up," Bolton instructed Hill to tell the lawyers, according to the Times.

Hill's lawyer, Lee Wolosky, said she received a congressional subpoena and would “comply and answer questions” from lawmakers on three committees investigating Trump's dealings with Ukraine: Foreign Affairs, Intelligence, and Oversight and Reform.

Her appearance came despite a White House vow to halt cooperation with what it termed the “illegitimate” impeachment probe. The White House did not immediately respond to questions about whether they had sought to limit Hill’s testimony.

More:What's ahead this week in the impeachment inquiry? A deadline for Rudy Giuliani, more Capitol Hill meetings

Hill was expected to face questions about the deepening scandal related to Trump urging that Ukrainian President Volodymyr Zelensky investigate his political rival, Joe Biden, while withholding military aid from that country.

Republicans have argued that Democrats are conducting impeachment behind closed doors and should release transcripts of depositions with Hill and others.

“The tragedy here and the crime here is that the American people don’t get to see what’s going on in these sessions,” said Rep. Jim Jordan of Ohio, the top Republican on the Oversight and Reform Committee.

But the chairman of the Intelligence Committee, Rep. Adam Schiff, D-Calif., said Sunday that hearing from witnesses behind closed doors prevents others from knowing what was said.

“We want to make sure that we meet the needs of the investigation and not give the president or his legal minions the opportunity to tailor their testimony and in some cases fabricate testimony to suit their interests,” Schiff said on CBS’ “Face the Nation.”

Rep. Matt Gaetz, R-Fla., tried to attend the deposition as a member of the Judiciary Committee. But Schiff and the House parliamentarian ruled that he couldn't attend because he wasn't a member of the three committees that scheduled the session.

“Traditionally, the Judiciary Committee has jurisdiction over impeachment,” Gaetz told reporters after being asked to leave. “If Adam Schiff and House Democrats were so proud of their work, they would be willing to show it.”

Hill left her post just before Trump's July call with Zelensky in which he pressed for the investigation, but NBC and CNN report she was was aware of the efforts by Rudy Giuliani, Trump's personal attorney, to find dirt on Biden in Ukraine.

NBC reported that Hill plans to tell congressional panels leading the impeachment effort that Giuliani and Gordon Sondland, the European Union ambassador who has been subpoenaed to appear before the panels, led a shadow policy on Ukraine that went around normal procedures and side-stepped the National Security Council.

After listing to Hill for hours behind closed doors, Rep. Jamie Raskin, D-Md., said each of the officials being questioned by the panels are offering pieces of a wider picture that is helping put together different aspects of the impeachment inquiry.

"All of these witnesses are filling in the picture of what was taking place because the famous — infamous July 25 telephone call was not some kind of one-off — it was the culmination of a continuing campaign by Rudy Giulian and his henchmen," he said, calling the Giuliani's work a "shadow" foreign policy campaign that aimed to "shakedown" the Ukrainian government.

Before helping guide the Trump administration on Russia and Euroasia policy, Hill was a Brookings Institution scholar, a frequent Putin critic and authored multiple books on Russia and Vladimir Putin, including "Mr. Putin: Operative in the Kremlin," which criticized the Russian president's ruling of the country. She holds master's degrees in Soviet studies, along with Russian and modern history, according to her Brookings Institution biography.

At least three other witnesses are expected to appear before the House Democrats' impeachment panel this week. Here's what you need to know about each:

• George P. Kent is an expert on Ukraine and Russia who now serves as a deputy assistant secretary at the State Department. He is scheduled to be deposed on Tuesday.

Kent landed in Giuliani’s crosshairs earlier this year, as the former New York mayor ramped up his pressure on the Ukrainian government to open investigations that could benefit Trump. In an interview with a Ukrainian news site in May, Giuliani alleged, without evidence, that Kent was working with George Soros, the liberal billionaire philanthropist, to find “dirty information” on Trump campaign officials.

Before taking his current post, Kent served as the deputy chief of mission in Kiev, and he also spent years working on anti-corruption efforts across Europe. He joined the foreign service nearly 30 years ago, and speaks Ukrainian, Russian and Thai, among other languages.

“He always struck me as a very professional person who was focused on trying to advance American interests,” said Michael McFaul, who served as the U.S. ambassador to Russia in the Obama administration.

• Gordon Sondland, Trump ambassador to the European Union, will return to Capitol Hill this week. The State Department blocked him from testifying last week. Sondland, A wealthy former hotel magnate from Oregon, he has emerged as a central player in the Ukraine affair.

Text messages released earlier this month show that he and Kurt Volker, Trump's former special envoy to Ukraine, orchestrated a months-long effort to push Ukraine's newly elected president, Zelensky, to make a public promise that he would order probes into Biden and Ukraine's alleged role in 2016 election meddling.

• T. Ulrich Brechbuhl is the State Department’s top lawyer and a long-time friend of Secretary of State Mike Pompeo. Pompeo and Brechbuhl both graduated from West Point in 1986, and they served together in Germany before the fall of the Berlin Wall.

Brechbuhl – who was born in Switzerland and speaks French, German and Swiss-German – came to the State Department after a long private-sector career. Before either Pompeo or Brechbuhl came to Washington, they were business partners at Thayer Aerospace, a Kansas defense company.

He joined the State Department in 2018 and has served as the agency’s counsel and senior adviser to Pompeo. Brechbuhl’s name first emerged in the Ukraine controversy last month, with the release of the explosive whistleblower complaint that alleged that Trump used the power of his office to solicit foreign interference in the 2020 election.

Brechbuhl was among those who listened to Trump’s July 25 phone call with Zelensky.

Contributing: Deirdre Shesgreen