The House impeachment probe shifted its focus Thursday to what Vice President Pence knew about the shadow campaign to compel Ukraine to open investigations that would benefit the Trump administration.

Efforts to fill out the picture have been stymied by the White House’s insistence that top administration officials defy congressional subpoenas. Former national security adviser John Bolton did not appear for scheduled testimony Thursday, and acting White House chief of staff Mick Mulvaney is expected to skip his hearing Friday.

That leaves Jennifer Williams, a special adviser to the vice president for Europe and Russia, as possibly the last witness in the month-long closed-door sessions of the impeachment investigation. She spent about four hours on Capitol Hill on Thursday.

Her lawyer, Justin Shur, said Wednesday that “Jennifer is a longtime dedicated State Department employee” and would answer investigators’ questions if required.

“We expect her testimony will largely reflect what is already in the public record,” Shur said in a statement.

Williams testified under a subpoena the House Intelligence Committee issued Thursday morning, after the White House tried to prevent her from attending the deposition. Nearly all of the administration officials to testify thus far have done so in compliance with subpoenas.

Williams is the first person from Pence’s staff to give testimony in the impeachment probe and was expected to field questions about the vice president’s interactions with Ukrainian leaders and any contact he may have had with Rudolph W. Giuliani, the president’s personal lawyer and point man for seeking investigations of Trump’s political rivals.

Initially, Pence was expected to lead an American delegation attending Ukrainian President Volodymyr Zelensky’s inauguration in May. But Trump ordered Pence to cancel those plans, U.S. officials said.

Williams’s testimony could shed further light on why Trump told Pence not to attend the inauguration. The presence of the vice president at the ceremony would have been an important sign of support for Zelensky, a political novice in a country mired in an internal war with Russian-backed forces occupying the country’s east.

When Pence was told to stay home, the White House dispatched Energy Secretary Rick Perry in his place.

Pence did meet with Zelensky in Warsaw on the sidelines of a World War II commemoration Sept. 1, after Trump pulled out to monitor a hurricane barreling toward Florida. The meeting came just days after the Ukrainians learned Trump had frozen $391 million in military aid earmarked for the country.

The money was important militarily and a sign to Russia of U.S. support to Ukraine in its long, stalemated war.

At the time, the Ukrainians were crestfallen that they were not going to be able to make their case for the aid directly to the president.

“Trump not going to Warsaw now. Pence going,” Kurt D. Volker, the administration’s then-special envoy for Ukraine, texted the country’s defense minister. “I’m so sorry.”

The message was among the internal communications turned over to impeachment investigators.

In his meeting with the Ukrainians, Pence was evasive regarding the reasons for the hold on aid, telling Zelensky and his team that Trump was eager to see them do more to tackle corruption and that he was frustrated that the European allies weren’t providing more support. He promised he would raise the issue of the frozen aid with the president that evening when he got back to Washington.

“It was not a strong statement of solidarity,” one Western diplomat said.

Williams would have briefed Pence ahead of his meeting with Zelensky. She also was one of a handful of U.S. officials on Trump’s controversial July 25 call with Zelensky in which Trump asked the Ukrainian leader to open an investigation into former vice president Joe Biden and his son Hunter, who landed a lucrative position serving on the board of an obscure Ukrainian gas company.

Officials close to Pence said he wasn’t aware of the demands Trump made of Zelensky on the call even though Pence likely received a rough transcript of the conversation in his nightly briefing book. As Pence’s top adviser on Ukraine matters, Williams would have been responsible for ensuring that Pence knew what happened on the call.