(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.

Read More