According to an official working on impeachment, David Holmes, who worked at the U.S. Embassy in Ukraine, is expected to testify in closed session on Friday. Mark Sandy, an official at the White House Office of Management and Budget (OMB), is expected to testify privately on Saturday.
Their attendance is not guaranteed. The committees have noticed depositions with administration officials who ultimately defied subpoenas seeking their testimony, citing White House orders blocking their appearances.
The announcement came just as members of the House were questioning two senior U.S. diplomats, William Taylor and George Kent, about the Trump administration's policy toward Ukraine.