Impeachment inquiry: Who is testifying before Congress this week?

A former White House adviser who oversaw the Russia and Ukraine policy for the Trump administration is expected to appear before Congress Monday as the impeachment inquiry into President Donald Trump's dealings with Ukraine continues

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Fiona Hill, who was a top advisor on the National Security Council in charge of Russia and Eurasia policy, is expected to testify that Rudy Giuliani, along with European Union Ambassador Gordon Sondland, circumvented the normal White House process "to pursue a shadow policy on Ukraine," according to reporting from NBC News.

Neither Hill nor her attorney has confirmed what she will be testifying to before House committees investigating the president and those around him.

Giuliani is the former mayor of New York City and Trump's personal attorney. Sondland, who is a wealthy former hotel magnate from Oregon, was blocked by the State Department from testifying before Congress last week. He is expected to testify Thursday.

The inquiry, which was announced by House Speaker Nancy Pelosi on Sept. 24, was launched after a whistleblower contacted members of Congress with concerns about a phone call Trump made in July to the president of Ukraine.

The phone call, according to Trump critics, had the president leveraging his power to hold $400 million in aid to Ukraine in exchange for information on former Vice President Joe Biden and his son Hunter Biden. Trump says the call congratulating Volodymyr Zelenskiy on becoming president of Ukraine was "perfect" and has been misinterpreted.

>> How does impeachment work? Here is the step-by-step process

Trump claims that Hunter Biden profited from an affiliation with a Ukrainian gas company and that Joe Biden had a prosecutor who was looking into the operations of the gas company fired to protect his son. Neither Biden has been officially accused of wrongdoing.

Scheduled to testify before House committees this week are:

George Kent, deputy secretary of state, oversees policy toward Ukraine in his State Department job. From 2015 to 2018, Kent was deputy chief of mission at the U.S. Embassy in Kyiv. Part of his job was to form anti-corruption policy in the region before 2015. He is scheduled to be deposed on Tuesday.

Sondland, according to the whistleblower's complaint, along with Volker, offered advice to the "Ukrainian leadership about how to "navigate" the demands that the President had made of Mr. Zelensky." Sondland and Volker also met with Giuliani, the whistleblower complaint said, to "attempt to contain the damage to U.S. national security" that Giuliani's efforts to find information on Biden was having.

Ulrich Brechbuhl, a State Department counselor, is also scheduled to testify this week. According to the whistleblower's complaint, Brechbuhl was on the July 25 phone call.