Eight witnesses are scheduled to testify this week in the impeachment investigation, including four on Tuesday.
Lt. Colonel Alexander Windman, Director of European Affairs at the National Security Council, and Jennifer Williams, Special Advisor to Vice President Pence for Europe and Russia, are scheduled to appear Tuesday morning.
Windman testified in a closed-door deposition last month that he "did not think it was right" for Trump to seek a Ukrainian investigation into a US citizen. He was among those who listened to the conversation between Trump and Ukrainian President Vladimir Zelensky on July 25. Later, Windman reported his concerns to NSC's lead lawyer.
Williams' closed-door testimony, which was released on Saturday, suggests that the Office of Management and Budget pressured Ukrainian military aid more than two weeks earlier than
Trump attacked Williams on Twitter on Sunday.
"Tell Jennifer Williams, anyone, to read MANY transcripts of presidential calls, and see the statement just issued from Ukraine," Trump said. "Then she has to meet the other Never Trumpers I don't know about & most of all I've never heard of and make a better presidential attack!"
Tim Morrison, top advisor to Russia and Europe on the Council on national security, and Kurt Volcker, a former envoy to the Trump administration in Ukraine, is scheduled to testify on Tuesday afternoon.
Morrison told House investigators last month that Gordon Sundland, an ambassador to the European Union, was acting at Trump's request and was talking to a top Ukrainian official about exchanging military aid for political investigations.
Trump stated that he did not know Sondland well and tried to distance himself from the EU ambassador whom Trump charged with Ukraine's policy along with two others, even though Ukraine is not part of the European Union.
Volker told impeachment investigators that he was working with Trump's personal lawyer Rudolf W. Giuliani, the White House and Ukrainian officials to arrange a July 25 phone call between Trump and Zelensky, while insisting on a White House visit about investigating Trump's political enemies.
Sundland is scheduled to testify Wednesday morning. Laura Cooper, Special Assistant Secretary of Defense for Russia and Ukraine, and David Hale, Secretary of State for Political Affairs, are scheduled this afternoon.
On Thursday, Fiona Hill, a former advisor to the National Security Council for Russia, is scheduled to appear.