This afternoon's testimony will come from two Trump Administration officials: Laura Cooper and David Hale.
Laura Cooper is the deputy assistant secretary of defense for Russian, Ukrainian and Eurasian affairs. In that position, she would have been aware of any holds or delays in delivering military aid to Ukraine. She testified in her closed door hearing that she was told there were "concerns about Ukraine and Ukraine security assistance," and was told the day after the July 25th telephone conversation that the aid was on hold, and the hold relates to his "concerns about corruption."
Cooper also testified that she drew a "very strong inference" that the Ukrainians understood that the funding was on hold after speaking to Kurt Volker. She also raised concerns in a meeting on July 26th that holding the aid could not be legally done.
David Hale is the undersecretary of state for political affairs. He is the 3rd-highest ranking official at the Department of State and was confirmed by the Senate in August 2018.
Hale testified in his deposition that he knew about the smear campaign against Marie Yovanovitch, and had been asked to be involved in releasing an official statement backing her up. He also testified that it was likely Pompeo who made the decision not to issue the statement.