WASHINGTON (AP) — House investigators are summoning top Trump officials, including former national security adviser John Bolton, to testify next week as the impeachment probe pushes ever deeper into the White House.

Investigators want to hear from Bolton, the hawkish former adviser who was openly troubled by President Donald Trump’s approach to Ukraine. He has since left the administration.

The Democrats are also calling John Eisenberg, the lawyer for the NSC who fielded an Army officer’s concerns over Trump, and Michel Ellis, another security council official. Eisenberg and Ellis are being asked to appear on Monday with Bolton on Thursday, according to a person familiar with the planning and granted anonymity to discuss it.

The White House has urged officials not to testify in the impeachment proceedings and it’s unclear if these people will appear before the House panels.

The impeachment inquiry, now in its second month, is accelerating and deepening as the House prepares to take its first official vote Thursday on the process. Then come public hearings and the possible drafting of articles of impeachment against the president.

The investigation is focused on Trump’s July phone call with Ukraine when he asked President Volodymyr Zelenskiy to investigate Democrats and a potential 2020 political rival, Joe Biden, as the White House was withholding military aid Ukraine relies on for its defenses.

READ MORE: Who’s who in the Trump impeachment inquiry?

On Wednesday, the Democratic and Republican House lawmakers heard fresh testimony about the Trump administration’s unusual back channels to Ukraine.

Two State Department Ukraine experts offered new accounts of Trump’s reliance on his personal attorney, Rudy Giuliani, rather than career diplomats, to engage with the East European ally, a struggling democracy facing aggression from Russia.

Foreign Service officer Christopher Anderson testified that Bolton cautioned him that Giuliani “was a key voice with the president on Ukraine” and could complicate U.S. goals for the country.

Another Foreign Service officer, Catherine Croft, said that during her time at the White House’s National Security Council, she received “multiple” phone calls from lobbyist Robert Livingston — a former top Republican lawmaker once in line to become House speaker — telling her the U.S. Ambassador to Ukraine, Marie Yovanovitch, should be fired.

“It was not clear to me at the time — or now — at whose direction or at whose expense Mr. Livingston was seeking the removal of Ambassador Yovanovitch,” she said in prepared remarks obtained by The Associated Press.

Livingston characterized Yovanovitch as an “‘Obama holdover and associated with George Soros,” she said, referring to the American financier who is often the subject of conservative criticism in the U.S. and Europe.

Most Democrats are expected to support the formal impeachment investigation resolution Thursday, even if they don’t back impeachment itself, saying they are in favor of opening the process with more formal procedures.

Public hearings are expected to begin in mid-November, a matter of weeks. Democrats are eager to hear from some top witnesses who have already provided compelling testimony behind closed doors, including diplomat William Taylor, a top ambassador in Ukraine, and Alexander Vindman, the Army officer who testified Tuesday that he twice reported to superiors his concerns about Trump’s actions toward Ukraine.

Vindman is willing to testify publicly, according to a person familiar with the situation and granted anonymity Wednesday to discuss it.

Trump and his Republican allies on Capitol Hill say the impeachment inquiry is illegitimate and are unpersuaded by the House resolution to affirm next steps.

At a Trump’s hotel in Washington, during a fundraiser for House Republicans and lengthy dinner afterward with GOP leaders, the president indicated he was prepared for the fight ahead, said those familiar with the private gatherings Tuesday night.

Senate Majority Leader Mitch McConnell said the format for the impeachment probe denies Trump the “most basic rights of due process.”

McConnell said that “instead of setting a high bar, House Democrats seem determined to set a new low.”

On Thursday, the investigators are to hear from Tim Morrison, a former top GOP aid on Capitol Hill, who served at Trump’s National Security Council and was among those likely monitoring the president’s call with Ukraine.

Both career diplomats testifying Wednesday had served as top aides to the former U.S. special envoy to Ukraine, Kurt Volker, who was the first to testify in the impeachment inquiry and whose cache of text messages provided key insight into Trump’s demands on the new Ukraine president.

Croft, who testified for nearly five hours, described being told at an administration meeting that security funds for Ukraine were being put on hold “at the direction of the president,” corroborating other accounts that have been provided to investigators.

In his opening statement, Anderson traced his unease with developments that he felt threatened to set back relations between the U.S. and Ukraine.

He told investigators that senior White House officials blocked an effort by the State Department to release a November 2018 statement condemning Russia’s attack on Ukrainian military vessels.

Both witnesses were instructed by the administration to not testify but appeared in response to subpoenas from the House, according to a statement from their attorney Mark MacDougall.

The lawyer told lawmakers that neither of his clients is the whistleblower whose complaint triggered the impeachment inquiry and that he would object to any questions aimed at identifying that person.

Associated Press writers Zeke Miller, Padmananda Rama, Matthew Daly, and Alan Fram contributed to this report.