(CNN) Former national security adviser John Bolton cautioned about the influence Rudy Giuliani had on US-Ukraine policymaking during a meeting in mid-June with top US officials, a career foreign service officer plans to tell Congress on Wednesday, according to a copy of his opening statement obtained by CNN.

Christopher Anderson, who was the assistant to former special envoy for Ukraine Kurt Volker from 2017-2019, is one of two State Department officials set to testify behind closed doors later Wednesday as part of the House Ukraine impeachment inquiry. Catherine Croft, who took over that role in summer 2019, is also testifying behind closed doors Wednesday, and the testimony started around 9:30 a.m. ET.

The House Intelligence Committee issued a subpoena to Croft on Wednesday morning for her testimony, according to an official working on the impeachment inquiry.

Volker, as well as a number of other State Department officials, have already testified as part of the probe. Neither witness is expected to unearth a treasure trove of details that will change the trajectory of the investigation, but their testimonies are expected give context for how the Trump administration's US-Ukraine policy developed and confirm details that have already been presented.

According to Anderson's prepared statements, he helped in May to develop "key deliverables" to show newly-elected Ukrainian President Volodymyr Zelensky's "commitment to reform. 1) demonstrating Zelenskyy's independence from powerful vested interests and pursuing anticorruption reform as well as antitrust reform; 2) strengthening U.S.-Ukrainian energy cooperation; and 3) improving our bilateral security relationship which included Ukraine increasing its purchases of key U.S. military equipment."

In the mid-June meeting, "Bolton stated that he agreed with our three lines of effort and that he also supported increased senior White House engagement," according to the prepared statement.

"However, he cautioned that Mr. Giuliani was a key voice with the President on Ukraine which could be an obstacle to increased White House engagement," Anderson is expected to tell the House Intelligence, Oversight and Foreign Affairs Committees.

Bolton, viewed as one of the key witnesses, has been contacted by the committees about a deposition. It is unclear if or when he will appear before the committees.

CNN has previously reported that Trump had directed Volker and other top US officials to deal with his personal layer on matters related to Ukraine, including the Ukrainian President's desire for a White House meeting -- an order was a clear circumvention of official channels.

Witnesses in the impeachment probe have testified to Giuliani's outsized influence on Ukraine policy, including pressing for an investigation into former Vice President Joe Biden and his son Hunter Biden's work with Burisma. There has been no evidence of wrongdoing by either Biden in Ukraine.

During his testimony in early October, Volker cast himself as an advocate of American support for Ukraine, but he was cognizant of the reality that Giuliani would have to be a part of anything if it were going to be successful, according to those familiar with his testimony.

Croft, who also served at the National Security Council as a Ukraine director from July 2017 to July 2018, will also shine light on pressure coming from outside allies of Giuliani to oust Marie Yovanvotich , now the former US ambassador to Ukraine, earlier than previously known.

"During my time at the NSC, I received multiple calls from lobbyist Robert Livingston, who told me that Ambassador Yovanovitch should be fired," Croft plans to say. "He characterized Ambassador Yovanovitch as an 'Obama holdover' and associated with George Soros. 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."

Livingston's lobbying firm has worked for Ukrainian politician Yulia Tymoshenko, a top but unsuccessful candidate in the country's most recent election, according to its federal foreign lobbying disclosure. In December 2018, the Livingston Group disclosed she was introduced to Giuliani.

It's unknown if Livingston's calls to the NSC were connected in any way to his work for the Ukrainian official or his firm's contact with Giuliani.

When reached on the phone by CNN on Tuesday evening, Livingston said he did not know about the witness statement regarding his opposition to Yovanovitch, and he declined to explain why he made the calls.

"I don't have any reason. Not going to talk about it. I don't know anything about it," he said, then hung up the phone.

Croft also plans to detail being on the July 18 video conference where an Office of Management and Budget official said that acting chief of staff and office head Mick Mulvaney put an "informal hold" on Ukraine security assistance.

"The only reason given was that the order came at the direction of the President," Croft is expected to say. She is also expected to tell the committees that she had heard about the hold before, but didn't recall precisely when she learned about it. The top US diplomat in Ukraine, Bill Taylor, was on that teleconference as well and told lawmakers during his deposition that he "sat in astonishment" when he heard the news.

Neither Croft nor Anderson were on the July 25 phone call when Trump pressed his Ukrainian counterpart to investigate Biden.

Anderson is also expected to testify that the Trump administration blocked a statement condemning Russian aggression toward Ukraine -- which was drafted by the State Department -- in November 2018. It came after Russia had seized Ukrainian military vessels in the Sea of Azov. The incident was a major escalation between the two former Soviet nations. Russia seized detained 24 Ukrainian sailors as part of the incident.

"While my colleagues at the State Department quickly prepared a statement condemning Russia for its escalation, senior officials in the White House blocked it from being issued," Anderson plans to say. "Ambassador Volker drafted a tweet condemning Russia's actions, which I posted to his account."

On November 26, Secretary of State Mike Pompeo issued a statement expressing "deep concern over the incident in the Black Sea that occurred November 25," saying it represented "a dangerous escalation and a violation of international law." Trump canceled a meeting with Russian President Vladimir Putin at the G-20 summit in Argentina days later over the incident.

Mark MacDougall, the lawyer who represents both witnesses, plans to introduce his clients to the House investigators.

MacDougall will tell the committees that neither of his clients are the whistleblower. He will also stress that Anderson and Croft will not answer questions about the whistleblower's identity.

"As the Committee is well aware, the governing stature and associated regulations permit whistleblowers to preserve their anonymity," MacDougall will tell lawmakers.

Anderson, who has been in the foreign service since 2005, also plans to say that he and his colleagues are "non-partisan."

Trump has repeatedly smeared the witnesses appearing before the congressional committees as "Never Trumpers" and "human scum." Many of the witnesses, including Anderson and Croft, are widely respected diplomats who have served in both Republican and Democratic administrations.

This story has been updated with additional developments Wednesday.