Washington — The diplomatic official who allegedly overheard President Trump ask about the status of "investigations" soon after his July phone call with the Ukrainian president is expected to appear before House lawmakers for a closed-door hearing on Friday.

David Holmes is the staffer referenced by diplomat Bill Taylor in testimony before the House Intelligence Committee on Wednesday, three sources familiar with the matter tell CBS News. Holmes is a counselor for political affairs at the U.S. embassy in Kiev, according to the embassy's website.

At the first public hearings of the impeachment inquiry, Taylor testified that a member of his staff, whom he did not name, was with U.S. Ambassador to the E.U. Gordon Sondland at a restaurant on July 26 when the ambassador placed a call to Mr. Trump.

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"The member of my staff could hear President Trump on the phone, asking Ambassador Sondland about 'the investigations,'" Taylor, the chargé d'affaires at the U.S. embassy in Kiev, said in his opening statement. "Ambassador Sondland told President Trump that the Ukrainians were ready to move forward."

Taylor said the staff member, now known to be Holmes, asked Sondland after the call what the president "thought about Ukraine."

David Holmes, a political officer at the U.S. embassy in Ukraine. Facebook

"Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for," Taylor said, indicating he was not aware of the conversation at the time of his deposition in October, and only learned about it last Friday.

White House press secretary Stephanie Grisham dismissed the testimony as "hearsay" on Wednesday. Robert Luskin, an attorney for Sondland, said his client "will respond when he testifies next week." The committee announced late Tuesday that Sondland was expected to testify in a public hearing next Wednesday, November 20.

William Burns, a career diplomat and former deputy secretary of state, described Holmes as "a very sharp, honest, experienced" foreign service officer, telling CBS News he was "one of the best I ever served with." Holmes served as Burns' special assistant on South and Central Asia from 2010 to 2011, before being detailed to the National Security Council as a director for Afghanistan during the Obama administration.

In addition to Holmes, an official working on the impeachment inquiry said Mark Sandy, a senior official from the Office of Management and Budget, has been asked to appear for a closed-door deposition on Saturday, November 16.

Holmes' expected closed-door testimony will fall on the same day as the second public hearing before the House Intelligence Committee, featuring testimony by former ambassador Marie "Masha" Yovanovitch.

Margaret Brennan contributed reporting.