Ambassador Philip Reeker is expected to appear in closed session before three Democrat-led House committees conducting an impeachment investigation into President Trump on Saturday, a congressional source told Fox News.

REPUBLICANS PRESS FOR WHISTLEBLOWER TESTIMONY, AND FOR ANSWERS ON A HEARING DELAY

Acting Assistant Secretary of State Philip Reeker is expected to appear in a closed session Saturday. (State Department)

Reeker’s testimony was originally scheduled for Thursday but members did not want to question the witness during a ceremony where the late Rep. Elijah Cummings, D-Md., will lie in state at the Capitol. Cummings’ funeral will be in Baltimore on Friday.

Rep. Jim Jordan, R-Ohio, who is the top-ranking Republican on the House Oversight Committee, wrote a letter to Reeker, the acting assistant secretary of state, demanding that the deposition be rescheduled to a business day to allow more GOP lawmakers to attend.

Jordan called on Reeker to explain the reasoning behind the rare Saturday deposition. He said he regrettably had to ask Reeker directly for the information because he has "no confidence" that Rep. Adam Schiff, as the leader of the impeachment inquiry, is "operating fairly or in good faith."

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Former Deputy Assistant to the President for National Security Affairs Charles Kupperman is expected to appear in a closed session on Monday and Timothy Morrison, a special assistant to the president, is expected to appear in a closed session next Thursday. The Committees are in ongoing discussions with other witnesses.

Fox News’ Chad Pergram contributed to this report.