U.S. Ambassador to Brazil Michael McKinley speaks during the first edition of the International Brazil Air Show at Tom Jobim international airport, known as Riogaleao, in Rio de Janeiro, Brazil, on March 29, 2017. / AFP PHOTO / YASUYOSHI CHIBA        (Photo credit should read YASUYOSHI CHIBA/AFP/Getty Images)
YASUYOSHI CHIBA/AFP/AFP/Getty Images
U.S. Ambassador to Brazil Michael McKinley speaks during the first edition of the International Brazil Air Show at Tom Jobim international airport, known as Riogaleao, in Rio de Janeiro, Brazil, on March 29, 2017. / AFP PHOTO / YASUYOSHI CHIBA (Photo credit should read YASUYOSHI CHIBA/AFP/Getty Images)
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(CNN) —  

Michael McKinley is testifying Wednesday behind closed doors as part of the House Democrats’ impeachment probe – less than a week after resigning as a senior adviser to Secretary of State Mike Pompeo.

McKinley, a former US ambassador, planned to open his testimony by telling investigators that his resignation was driven by his concern that the Department’s leadership was not supporting career foreign service officers, according to a source familiar. In the first hour of the testimony, McKinley specifically raised concerns about the removal of Ambassador Marie Yovanovitch from Ukraine, and how the State Department did little to protect her, said a source familiar with his remarks.

He arrived on Capitol Hill Wednesday morning for an appearance before the House Intelligence, Oversight and Foreign Affairs committees. The former State Department adviser is appearing for a transcribed interview, according to one source, which indicates he is not coming under subpoena.

McKinley grew close with Pompeo when he was US ambassador to Afghanistan early on in the Trump administration, when Pompeo was the director of the CIA. Pompeo recruited him to work closely with him when he began at State, in an effort to recruit a career official he trusted who had a close relationship with career foreign service officers.

McKinley started to grow increasingly weary when a damning State Department Inspector General report came out in late August documenting alleged inappropriate leadership practices of Assistant Secretary Kevin Moley, a Trump appointee. McKinley viewed the decision not to fire Moley as “not only wrong, but unconscionable,” the source explained. Moley, a Trump appointee, is still in his post leading the bureau of international organization affairs at the State Department. The report from the State Department inspector general said that approximately 50 of 300 domestic employees in Moley’s bureau have departed since he took over its leadership in 2018, nearly all of them saying poor leadership contributed to their decision to depart. Moley replied to the report saying the information it contained was concerning, but claimed there was contextual information missing and factual inaccuracies.

McKinley declined to comment ahead of his testimony.

CNN reported on Thursday that the longtime diplomat was leaving his post as a top adviser to the secretary of state – a role he had held since May 2018. His resignation and congressional testimony come as the State Department faces increased pressure from House investigators and as numerous current and former State Department officials have expressed fear and outrage over the department’s handling of the Ukraine scandal.

McKinley was deeply concerned with the silence in the top ranks at State in not defending former US Ambassador to Ukraine Marie “Masha” Yovanovitch, and it was one reason he resigned, one source told CNN. The source said that McKinley had been mulling over the decision for a few weeks.

A former senior State Department official told CNN that McKinley was “known to be a man of integrity, a man of principle.”

“If he had encountered something either that was happening in the State Department or he felt he could no longer carry out his duties without compromising his integrity and his principles, he is somebody who would feel he had no choice but to resign,” they said.

Decades in foreign service

McKinley had been in the foreign service for more than three decades and served as ambassador to Peru, Colombia, Afghanistan and Brazil.

In a farewell email to his State Department colleagues obtained by CNN on Friday, McKinley wrote that he was “leaving the department to pursue other opportunities, wherever they might lead.”

“The decision is personal: it’s time after 37 years with the Department,” he wrote.

Asked about McKinley’s departure in an interview in Nashville on Friday, Pompeo said his adviser “wanted to go on and begin the next phase of his life.”

“You know I don’t talk about personnel matters, but I’ve known Mike McKinley for quite some time, and he’s been with the State Department for 37 years. He told me for lots of good and sufficient reasons for him and his family he wanted to go on and begin the next phase of his life,” he told WKRN-TV, a CNN affiliate.

Pompeo recruited McKinley as an adviser and liaison to the career service. The career diplomat was a trusted adviser and traveled frequently with Pompeo on official trips.

This story has been updated with additional developments Wednesday.

CNN’s Manu Raju, Jeremy Herb, Jamie Gangel and Caroline Kelly contributed to this report.