WASHINGTON (AP) — Fiona Hill, a former White House adviser on Russia, told House impeachment investigators behind closed doors that she had strongly and repeatedly objected to the ouster earlier this year of former ambassador to Ukraine Marie Yovanovitch, according to a person familiar with the testimony.

Yovanovitch previously testified that President Donald Trump pressured the State Department to fire her.

Hill made the remarks on Monday as she testified for more than 10 hours in the Democratic inquiry, which is probing Trump's pleas to Ukrainian officials for investigations into political rival Joe Biden's family and into the country's involvement in the 2016 presidential election. The person requested anonymity to discuss the confidential interview.

The interview is one of what could eventually become dozens of closed-door depositions in the impeachment probe. There are five more scheduled this week, mostly with State Department officials, though it is unclear if they will all appear after Trump declared he wouldn't cooperate with the probe.

While interviews have focused on the interactions with Ukraine, the probe could broaden as soon as next week to include interviews with White House budget officials who may be able to shed light on whether military aid was withheld from Ukraine as Trump and his lawyer Rudy Giuliani pushed for the investigations.

The three committees leading the probe are seeking interviews next week with Russell Vought, acting director of the Office of Management and Budget, and Michael Duffey, another OMB official who leads national security programs, according to a person familiar with those requests. That person wasn't authorized to discuss the invitations and requested anonymity.

The packed schedule of interviews comes as Democrats are methodically working to pin down the details of Trump's pressure on Ukrainian President Volodymyr Zelenskiy. Once Democrats have completed the probe and followed any other threads it produces, they will use their findings to help determine whether to vote on articles of impeachment. House Speaker Nancy Pelosi said she wants the committees to move "expeditiously."

Democrats have already obtained documents and testimony that verify parts of an original whistleblower's complaint that launched the probe. A cache of text messages between three diplomats provided by one of the inquiry's first witnesses, former Ukrainian envoy Kurt Volker, detailed attempts by the diplomats to serve as intermediaries around the time Trump urged Zelenskiy to start the investigations into a company linked to Biden's son. Yovanovitch told lawmakers on Friday that there was a "concerted campaign" against her based on "unfounded and false claims by people with clearly questionable motives."

One of the diplomats in the text exchanges, U.S. ambassador Gordon Sondland, is expected to appear for a deposition under subpoena Thursday. He's expected to tell Congress that his text message reassuring another envoy that there was no quid pro quo in their interactions with Ukraine was based solely on what Trump told him, according to a person familiar with his coming testimony.

Also up this week: Michael McKinley, a former top aide to Secretary of State Mike Pompeo who resigned last week. McKinley, a career foreign service officer and Pompeo's de facto chief of staff, resigned Friday, ending a 37-year career. He is scheduled to testify behind closed doors Wednesday.

The committees are also scheduled to talk to Deputy Assistant Secretary of State George Kent on Tuesday and Ulrich Brechbuhl, a State Department counselor, on Thursday. On Friday, the lawmakers have scheduled an interview with Laura Cooper, who is the Deputy Assistant Secretary of Defense for Russia, Ukraine and Eurasia. It is unclear if any of those officials will show up after Trump's vow of non-cooperation.

Because of the Trump administration's edict, the Democrats have been subpoenaing witnesses as they arrived for their interviews — a move sometimes known as a "friendly" subpoena that could give the witnesses additional legal protection as they testify. Both Yovanovitch and Hill received subpoenas the mornings of their testimony.

One witness who may not be called before Congress is the still-anonymous government whistleblower who touched off the impeachment inquiry.

Top Democrats say testimony and evidence coming in from other witnesses, and even the Republican president himself, are backing up the whistleblower's account of what transpired during Trump's July 25 phone call with Zelenskiy. Lawmakers have grown deeply concerned about protecting the person from Trump's threats and may not wish to risk exposing the whistleblower's identity.

House Intelligence Committee Chairman Adam Schiff, D-Calif., said Sunday it "may not be necessary" to reveal the whistleblower's identity as the House gathers evidence. He said Democrats "don't need the whistleblower, who wasn't on the call, to tell us what took place on the call."

Schiff said the "primary interest right now is making sure that that person is protected."

Trump showed no signs of backing down.

"Adam Schiff now doesn't seem to want the Whistleblower to testify. NO!" the Republican president tweeted Monday. "We must determine the Whistleblower's identity to determine WHY this was done to the USA."

Republican lawmakers have aimed their ire at Democrats and the process, saying Pelosi should hold a vote to begin the inquiry and hold the meetings out in the open, not behind closed doors.

"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 Ohio Rep. Jim Jordan, the top Republican on the House Oversight and Reform panel.

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Associated Press writers Eric Tucker and Lisa Mascaro contributed to this report.