President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's former top assistant working on European and Russian issues will reportedly testify to Congress that Trump's top lawyer, Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, and his associates set up a shadow foreign policy for the Trump administration relating to Ukraine that circumvented the National Security Council (NSC).

NBC News reported Thursday that Fiona Hill, a former special assistant to the president, will tell House lawmakers that Giuliani and Trump's ambassador to the European Union, Gordon Sondland, went around the NSC and official White House procedures to speak to the president about Ukraine.

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Hill's attorney did not immediately return a request for comment from The Hill.

Her impending testimony is said to have caused worry in the White House, as Hill's tenure as one of Trump's top advisers stretches to the earliest months of his presidency.

Hill left the Trump administration in June after serving under two national security advisers, H.R. McMaster and John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, the latter of whom was ousted from the Trump administration last month.

Considered by many to be a top expert on Russian President Vladimir Putin Vladimir Vladimirovich PutinWatchdog confirms State Dept. canceled award for journalist who criticized Trump Former intelligence agency director Robert Cardillo speaks out against 'erratic' Trump Kremlin: Putin calls for reset between US and Russia on cyber relations before elections MORE, Hill is the first former White House official to agree to comply with House Democrats' impeachment inquiry.

The probe was launched in response to a controversial conversation between the president and Ukraine's President Volodymyr Zelensky during which Trump pressed Zelensky to investigate former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, a top 2020 Democratic contender for president.

The White House has opted to use executive privilege to stop officials from testifying before Congress, but it remains legally unclear if the president can force former officials who are now private citizens from testifying.