Fiona Hill, President Trump’s former national security adviser on Russian and European affairs, is testifying Monday before Congress in the impeachment inquiry against Mr. Trump. Here’s what to know about Hill:

She is expected to tell lawmakers that President Trump’s personal attorney, Rudy Giuliani, and Ambassador to the European Union Gordon Sondland actively avoided her and the National Security Council process and ran their own Ukraine policy, a source familiar with Hill told CBS News.

Hill, who is testifying voluntarily, does not intend to hand over documents or texts. She is the first individual who worked in the White House who is testifying in the impeachment inquiry. The former U.S. special envoy to Ukraine, Kurt Volker, was deposed last week, and the former U.S. ambassador to Ukraine, Marie Yovanovitch, testified Friday.

Yovanovitch, in her deposition, recounted the circumstances of her ouster as ambassador and denied the accusations promulgated by Giuliani and other Trump allies — that she had been “disloyal” to the president and had tried to obstruct Ukrainian investigations of corruption. She indicated that Giuliani associates “may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine.”

She’s currently on a leave of absence from the Brookings Institution in Washington, D.C., where she is the director for the Center on the United States and Europe. She officially left the White House in July, and her departure came before the July 25 call between Mr. Trump and Ukrainian President Volodymyr Zelensky took place.

Hill was born in northern England and studied at the University of St. Andrews in Scotland and Harvard.

Hill is an authority on Russia and has written a book on the Russian president, called “Mr. Putin: Operative in the Kremlin.” She has also written two other books on Russia: “The Siberian Curse: How Communist Planners Left Russia Out in the Cold,” and “Energy Empire: Oil, Gas and Russia’s Revival.”

— Margaret Brennan contributed to this article

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