A top Pentagon official arrived at the Capitol Wednesday to provide closed-door testimony related to 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 contacts with the Ukraine, the latest witness deposition as Democrats charge forward with their impeachment inquiry.

Laura Cooper, the deputy assistant secretary of defense for Russia, Ukraine and Eurasia, did not answer questions from reporters as she entered the hearing room where both current and former officials have testified in recent weeks about alleged efforts by Trump and his personal attorney, 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, to press Ukrainian President Volodymyr Zelensky to launch investigations into both interference in the 2016 presidential election and 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.

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Her testimony comes a day after William Taylor offered what Democrats described as some of the most damaging testimony yet — in which the top diplomatic official to Ukraine linked the administration's decision to withhold roughly $400 million in aid to demands for politically motivated probes. Republicans have sought to downplay Taylor's testimony, saying he relayed mostly secondhand information as it related to an alleged quid pro quo.

Cooper, who has worked in the federal government for almost two decades, was initially slated to testify Friday, but her deposition was moved at least two times as Democrats shuffled the order of witnesses.

Democrats are hoping she can shed light on any role the Pentagon played in withholding aid to Ukraine.

Some Democrats have said witnesses like Cooper are helpful for building a deeper understanding of how things work behind the scenes with the administration's Ukraine policy.

“As a prosecutor, what we would do is we would sort of interview as many witnesses as we can before we ask major players for interviews,” Rep. Ted Lieu (D-Calif.) said last week. “That's generally what you would do in an investigation."