WASHINGTON — Fiona Hill, who was until recently President Donald Trump’s top aide on Russia and Europe, plans to tell Congress that Rudy Giuliani and E.U. ambassador Gordon Sondland circumvented the National Security Council and the normal White House process to pursue a shadow policy on Ukraine, a person familiar with her expected testimony told NBC News.

Hill’s appearance next week before Congress has stoked fear among people close to the president, said a former senior White House official, given her central role overseeing Russia and Ukraine policy throughout most of the Trump administration.

Her plans to testify also pose a key test for whether congressional committees pursuing an impeachment inquiry can obtain testimony from other former officials who have left the administration, given the possibility that the White House may try to assert executive privilege to stop them from testifying.

Hill plans to say that Giuliani and Sondland side-stepped the proper process for accessing Trump on Ukraine issues, the person familiar with her expected testimony said, including circumventing John Bolton, who was Trump’s national security adviser until September.

Text messages recently released by Congress showed Sondland, Giuliani and former U.S. envoy for Ukraine Kurt Volker working to facilitate Trump’s goal of getting Ukrainian President Volodymyr Zelenskiy to commit to investigate the president’s political opponents including former Vice President Joe Biden — and making a White House visit for Zelenskiy contingent on such a commitment. Official notes from Trump’s call with Zelenskiy released by the White House showed Trump asking the Ukrainians to work directly with Giuliani, and NBC News has reported that Sondland was also in direct contact with Trump about Ukraine.

Hill, through her attorney Lee Wolosky, declined to comment.

Sondland’s attorney didn’t immediately respond to a request for comment. But Giuliani said he doesn't know Hill. He defended his actions on Ukraine by saying his only policy discussions on Ukraine had come at the request of the State Department and of Andrey Yermak, a top aide to Zelenskiy.

"Maybe she was engaged in secondary foreign policy if she didn’t know I was asked to take a call from President Zelenskiy’s very close friend," Giuliani told NBC News late Thursday. "I don’t know she knows what happened. She certainly never asked me. This is so far from a search for the truth it’s a joke or a very sad commentary on what we have let ourselves become."

A Russia hawk and one of the U.S. government’s foremost experts on Russian President Vladimir Putin, Hill had wound down her role overseeing Russia and Europe policy at the National Security Council in July shortly before Trump’s call with Zelenskiy. So it’s unclear how much she would know about that call, although she would have had deep knowledge of events leading up to the call.

At the White House, Hill was not viewed as a Trump loyalist, leading those close to Trump to worry that she can’t be controlled or pressured not to reveal potentially damaging information about the president, the former senior White House official said.

Congressional committees this week requested that Hill testify on Oct. 14, according to a letter obtained by NBC News, and she agreed. She has not been subpoenaed by Congress.

So far, the White House has not contacted her to assert privilege and try to prevent her from testifying, the person familiar said. But given the prospect that may occur, it’s possible that Hill may ultimately testify under subpoena from Congress, which would compel her to testify and serve as a powerful argument for her to comply with the request from Congress even if the White House does try to stop her.

The White House has told House Speaker Nancy Pelosi that it won’t cooperate with the impeachment inquiry as it currently stands. But while the White House can assert executive privilege— the right of the president to withhold some deliberative information from other branches of government— over current officials, the law is far murkier when it comes to private citizens who no longer work for the government.

If Hill does testify as planned, it could strengthen House Democrats’ hand to pursue testimony from many other former White House officials who would have insight into activities central to the impeachment inquiry— including from former officials who may be disgruntled. That could include Bolton and his former aides at the National Security Council who also departed when Trump fired Bolton last month.

Current officials are employed by the executive branch, so an assertion of privilege is essentially an instruction that employees would be hard-pressed to ignore. Private citizens have no similar obligation and the assertion of privilege could run up against their First Amendment rights.

Earlier this year the White House tried to assert the privilege over former White House Counsel Don McGahn, and the Justice Department’s Office of Legal Counsel issued a legal opinion arguing the privilege applies to former officials. But the argument has rarely been tested in court and remains an unsettled legal issue. In August, the House Judiciary Committee sued to try to compel McGahn to testify.