WASHINGTON — Fiona Hill, a former top National Security Council expert on Russia, testified to Congress behind closed doors Monday, the latest former Trump administration official to be subpoenaed as part of the House impeachment inquiry into President Donald Trump.

Hill wouldn’t comment as she arrived on Capitol Hill, but the hearing was stretching beyond 10 hours in the closed session. Her attorney said she had received a congressional subpoena and would “comply and answer questions” from lawmakers. She resigned from the White House National Security Council over the summer.

She is the first White House official to appear as part of the House impeachment inquiry.

A former top aide to Secretary of State Mike Pompeo has also been asked to appear for an interview this week, according to several officials familiar with the planning.

Michael McKinley, a career foreign service officer and Pompeo’s de facto chief of staff, resigned Friday, ending a 37-year career, as the impeachment probe turns its focus on the State Department in the Ukraine matter.

The sources were unauthorized to discuss the planning and were granted anonymity. House Democrats asked that McKinley appear for a closed-door interview Wednesday, the day after George Kent, another State Department official, is scheduled. It is unclear if they will appear.

Republicans called on Rep. Adam Schiff, the chairman of the House intelligence committee, to release transcripts of the depositions to the public. The California Democrat said Sunday that having witnesses appear behind closed doors would prevent them from knowing what other witnesses said.

“We want to make sure that we meet the needs of the investigation and not give the president or his legal minions the opportunity to tailor their testimony and in some cases fabricate testimony to suit their interests,” Schiff said on CBS’ “Face the Nation.”

Rep. Jim Jordan, the top Republican on the House oversight committee, said he learned Monday morning that Schiff had subpoenaed Hill.

“She was going to come, she’d agreed to come, she was going to come voluntarily but he’s going to subpoena her, I believe, so he could ask certain questions and again keep those secret except for the certain things he wants to leak,” Jordan said. “The tragedy here and the crime here is that the American people don’t get to see what’s going on in these sessions.”

Later this week, U.S. ambassador Gordon Sondland, Trump’s hand-picked ambassador to the European Union, is expected to appear for a deposition against the wishes of the White House, under subpoena. Sondland’s appearance, is set for Thursday.

The White House dismisses the impeachment inquiry as “illegitimate” because there has been no formal vote of the House to open impeachment proceedings.