Tim Morrison, a top White House Russia expert, arrived Thursday to testify behind closed-doors about 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 Ukraine, the latest in a growing string of witnesses to buck the president and appear as part of the Democrats’ impeachment investigation.

Morrison is expected to be asked about whether the president and other officials sought to withhold nearly $400 million in financial aid to Ukraine in exchange for a public commitment from Ukrainian President Volodymyr Zelensky to open two investigations that would benefit Trump politically.

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Morrison was escorted into the Capitol roughly 15 minutes before the scheduled 8 a.m. deposition. He declined to answer questions from reporters.

Morrison, who announced Wednesday that he would be leaving his position as the senior director for European and Russian affairs on the National Security Council (NSC), has been mentioned by previous witnesses as someone who voiced dismay that the president was demanding a quid pro quo for the aid.

Notably, Bill Taylor, a top diplomat to Ukraine, testified earlier this month to the three House committees leading the impeachment inquiry that Morrison made contact with NSC lawyers regarding the Trump administration allegedly demanding Ukrainian officials 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 and his son Hunter Biden.

Taylor also said Morrison described a “sinking feeling” to him in early September after Trump told U.S. Ambassador to the European Union Gordon Sondland that there was no “quid pro quo,” all while continuing to insist that Zelensky should publicly announce that he is “opening investigations of Biden and the 2016 election interference.”

Taylor also testified that Morrison, in another conversation, expressed concern that Trump didn’t want to give any assistance to Ukraine, which is fighting an insurgency of Russian-backed separatists in eastern parts of the country.

“That was extremely troubling to me,” Taylor said, according to his prepared remarks. “If the policy of strong support for Ukraine were to change, I would have to resign. Based on my call with Mr. Morrison, I was preparing to do so.”

Sondland, who testified shortly before Taylor, told investigators that Trump in a phone call insisted there was no quid pro quo for Ukraine aid. The wealthy hotelier also denied knowledge that the president’s requests for an investigation into Burisma Holdings, a Ukrainian energy company where Hunter Biden was employed, was an effort to damage a 2020 presidential rival.

Still, others have contradicted him, describing Sondland as deeply involved in pressing Zelensky for politically motivated probes, in exchange for setting up a White House visit between Trump and Zelensky as well as releasing aid to Ukraine.

Lt. Col. Alexander Vindman, the director for European affairs on the NSC, testified Tuesday that during a July 25 phone call, Sondland “started to speak about Ukraine delivering specific investigations in order to secure the meeting with the President,” which led then-national security adviser 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 to cut the meeting short.

“Following this meeting, there was a scheduled debriefing during which Amb. Sondland emphasized the importance that Ukraine deliver the investigations into the 2016 election, the Bidens, and Burisma,” according to Vindman’s opening remarks. “I stated to Ambassador Sondland that his statements were inappropriate.”

Morrison is also believed to have coordinated with the White House to set up a phone call between Trump and Zelensky — an exchange he will likely be pressed on when he testifies before the House Intelligence, House Oversight and Reform, and House Foreign Affairs committees.

Bolton recruited Morrison to serve as the senior director for European and Russian affairs on the NSC, a role previously held by Fiona Hill, who already testified earlier in the month about Ukraine as part of Democrats’ impeachment inquiry.

His departure from the White House comes after roughly 15 months on the job.

It is unclear whether Morrison was asked to leave his position. Some reports indicate that his departure was planned after Bolton resigned from his post in September, following conflicts with the president on key foreign policy issues such as Iran and North Korea.

NPR first reported his departure and that Andrew Peek, the deputy assistant secretary of State for Iraq and Iran, will be taking over his role.

“After more than a year of service at the National Security Council, Mr. Morrison has decided to pursue other opportunities — and has been considering doing so for some time,” a senior administration official said in a statement Wednesday. “We wish him well.”

The House Intelligence Committee, as it has done with prior witnesses, subpoenaed Morrison in order to combat attempts by the White House to block or limit his testimony, according to an official working on the impeachment inquiry.

"The House Intelligence Committee issued a subpoena to compel his testimony this morning. As is required of him, Mr. Morrison is complying with the subpoena and answering questions from both Democratic and Republican Members and staff," according to an official working on impeachment.

Updated at 8:56 a.m.