(CNN) An aide to Vice President Mike Pence -- who was concerned about the July 25 call between President Donald Trump and the Ukranian President -- is testifying behind closed doors Thursday, the latest deposition for the Democratic-led House impeachment inquiry.

An official working on the impeachment inquiry told CNN that Jennifer Williams was issued a subpoena Thursday morning for her deposition, "in light of an attempt by the White House to direct" her "not to appear."

Williams was on the July 25 call between Trump and Ukraine President Volodymr Zelensky , and she was concerned about what she heard on the call but there is no indication Williams raised her concerns to her superiors, according to the source.

Justin Shur, Williams' attorney, told CNN in a statement Wednesday night that she would answer the committee's questions "if required to appear."

"Jennifer is a longtime dedicated State Department employee," Shur said in the statement. "If required to appear, she will answer the Committees' questions. We expect her testimony will largely reflect what is already in the public record."

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