Fiona Hill, who until August served as President Donald Trump's top Russia analyst, testified under subpoena on Monday to the House as part of the impeachment inquiry into the president.

Previously, she agreed to testify at Congress' written request.

In her closed-door testimony, Hill planned to tell Congress that Trump's personal attorney Rudy Giuliani and U.S. Ambassador to the European Union Gordon Sondland circumvented the administration to pursue a shadow foreign policy on Ukraine, a person familiar with her expected testimony told NBC News last week. A former senior White House official told NBC News that Hill's appearance has caused concern among those close to Trump because she played a central role in the administration's Russian and Ukrainian policy.

Hill's testimony comes after Marie Yovanovitch, the ousted U.S. ambassador to Ukraine, told House investigators last week that Trump pressured the State Department to remove her. Pushing for Yovanovitch's ouster was central to an effort from two Soviet-born business associates of Giuliani who now face federal charges over campaign-finance violations.

And Hill's testimony comes before Sondland is set to testify Thursday that he did not know why congressionally approved military aid to Ukraine was withheld, a person with knowledge of his testimony told NBC News. Sondland is set to say he "relied" on Trump's pledge that there was no quid pro quo with regard to Ukraine investigating the Biden family and a conspiracy theory involving the 2016 presidential election when Sondland texted the current top diplomat in Ukraine last month saying no quid pro quo was taking place.