WASHINGTON (AP) — The swift-moving impeachment probe pushed onward Wednesday as a former top State Department aide testified that the Trump administration’s politicization of foreign policy contributed to his resignation, while the Senate GOP leader briefed colleagues on a possible Christmas impeachment trial.

The day’s events, interrupted by an explosive meeting at the White House, churned as longtime State Department officials are speaking out under subpoena — some revealing striking new details — about the actions Trump, and his personal lawyer Rudy Giuliani, took toward Ukraine that have sparked the House impeachment inquiry.

On Wednesday, Michael McKinley, a career foreign service officer and Secretary of State Mike Pompeo’s de facto chief of staff, told investigators behind closed doors that he could no longer look the other way amid the Trump administration’s dealings with Ukraine, which were among the reasons he ended his 37-year career last week, according to multiple people familiar with the testimony, who, like others who spoke to The Associated Press on condition of anonymity, were not authorized to discuss it.

“I was disturbed by the implication that foreign governments were being approached to procure negative information on political opponents,” McKinley testified, according to a former colleague familiar with his remarks.

The impeachment inquiry revolves around a whistleblower’s complaint that Trump was pushing Ukraine’s leader into opening an investigation of a company connected to the son of Trump’s potential 2020 Democratic rival Joe Biden. It is illegal to solicit or receive foreign help in a U.S. election.

Among McKinley’s concerns was the administration’s failure to support Ukrainian Ambassador Marie Yovanovitch, who was ousted in March on orders from Trump.

McKinley, who as a Latin America expert was not specifically involved in Ukraine, was also frustrated that there had been no response to an August inspector general’s report that found significant evidence of leadership and management problems, including allegations from career employees that Assistant Secretary of State Kevin Moley and his former senior adviser Marie Stull retaliated or tried to retaliate against them as holdovers from the Obama administration.

Rep. Mark Meadows, R-N.C., told reporters outside the closed-door hearing that McKinley was complimentary about Pompeo’s role but did raise other issues.

“I think most of this is a concern by a colleague for an ambassador that he held in high regard,” Meadows said, declining to provide more details of the closed session.

Republicans are crying foul over the process of the impeachment inquiry, but as House Democrats press on with the investigation, Senate Majority Leader Mitch McConnell briefed Republicans about the possible trial ahead.