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.