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The investigation, which was announced a week ago, was launched after a whistleblower contacted members of Congress with concerns about a phone call Trump made in July to the president of Ukraine.
Here’s a list of who will be making their way to Capitol Hill to talk to legislators.
Thursday: Kurt Volker, who was a special representative for Ukraine, arranged meetings between President Donald Trump's attorney Rudy Giuliani and the representatives of Ukrainian President Volodymyr Zelensky, according to the whistleblower's complaint.
Monday: George Kent, the deputy assistant secretary in the European and Eurasian Bureau at the State Department, will be deposed next Monday. He oversaw policy toward Ukraine in the State Department.
Tuesday T. Ulrich Brechbuhl, who works as the counselor of the State Department, provides guidance to the secretary of state on foreign policy. Brechbuhl was listening in on the Ukraine call, according to the whistleblower complaint.
Oct. 10: Ambassador Gordon Sondland, ambassador to the European Union, along with Volker met with Giuliani to try to "contain the damage" Giuliani's efforts to gather information on Biden were having on U.S. national security.
Unscheduled: Marie "Masha" Yovanovitch, a former ambassador to Ukraine, was to have been deposed Wednesday, but that deposition has been delayed until sometime next week.
Mike Pompeo, secretary of state
The chairmen of the Foreign Affairs, Intelligence and Oversight and Reform committees subpoena Secretary of State Mike Pompeo for documents about the July phone call to Zelensky. Pompeo was listening to the call in July.
Rudy Giuliani, one of Trump’s personal attorneys
Giuliani has also been subpoenaed by the same three committees for documents related to his interactions with Ukrainian officials.
According to a memo from Rep. Elijah Cummings, D-Maryland, who is the chairman of the House Oversight Committee, the White House’s “flagrant disregard of multiple voluntary requests for documents — combined with stark and urgent warnings from the Inspector General about the gravity of these allegations — have left us with no choice but to issue this subpoena.” The subpoena for documents relating to the call to Zelensky is expected to be issued Friday. The documents that are being requested were first asked or on Sept. 9, Cummings said.
Thursday: Michael Atkinson, the inspector general for the intelligence community, is scheduled for a hearing on Thursday. Atkinson was the person who received the whistleblower's complaint. Atkinson deemed it "credible and urgent." The hearing will take place behind closed doors.