A top aide to Vice President Mike Pence was expected to testify Thursday in the House impeachment inquiry into President Trump.

Jennifer Williams, a special adviser to Pence for Europe and Russia, and a career Foreign Service officer, would be the first person from the veep’s office to testify in the Democrat-led impeachment probe, according to The Washington Post, which first reported the story.

She’s scheduled for the same day as ex-national security adviser John Bolton, who is not expected to comply with the request to appear.

Williams accompanied Pence to Warsaw in September where he met with Ukraine President Volodymyr Zelensky to talk about the nearly $400 million in military aid that Trump had put on hold as he asked Zelensky for a probe into Joe Biden and his son Hunter, as well as Ukraine’s supposed role in the 2016 election.

Her testimony could reveal some of what Pence knew about the hold on the Ukrainian aid and efforts by Trump and Rudy Giuliani to pressure the Ukrainians to launch probes into the Bidens, whom the president has accused of corruption while offering no evidence.

In his meeting with Zelensky, Pence said that the aid to Ukraine was not going to be delivered over concerns that the country had not done enough to fight corruption, even as it was battling Russian-backed separatists in the nations’s east.

Pence’s supporters say he was unaware that the president was digging for dirt on the Bidens at the time of the meeting.