Washington (CNN) A US diplomat who overheard President Donald Trump ask the US Ambassador to the EU about the status of "investigations" during a cellphone conversation in a Kiev restaurant is set to testify publicly in the House impeachment inquiry on Thursday.

David Holmes, the counselor for political affairs at the US Embassy in Ukraine, overheard the conversation between Trump and Gordon Sondland the day after Trump spoke with the Ukrainian president by phone in July, CNN previously reported . Holmes heard Trump ask Sondland on the call if the Ukrainians were going to "do the investigation," and Sondland responded, "He's gonna do it."

He told lawmakers in his closed-door deposition on November 15 that he'd "never seen anything" like the call Sondland placed to Trump at the restaurant.

"This was an extremely distinctive experience in my Foreign Service career," Holmes said, according to a transcript of his deposition that was released Monday night. "I've never seen anything like this, someone calling the President from a mobile phone at a restaurant, and then having a conversation of this level of candor, colorful language. There's just so much about the call that was so remarkable that I remember it vividly."

This confirms testimony from Bill Taylor , the top US diplomat in Ukraine, who revealed the July 26 phone call with lawmakers at his public testimony last week , saying Holmes had informed Taylor about it after he first appeared on Capitol Hill behind closed doors last month.

Holmes is a career foreign service officer who arrived in Ukraine in 2017, according to a source who knows him and describes him as a "sharp guy." He joined the foreign service in 2002, according to the American Foreign Service Association, and has previously served in Kabul, New Delhi, Kosovo, Bogota, Moscow and Kosovo.

Holmes has also served as a special assistant for South and Central Asia to former Under Secretary of State for Political Affairs Bill Burns and spent time on the National Security Council staff at the White House as director for Afghanistan from 2011 to 2012.

"He is a decent guy and a smart, competent foreign service officer," said a second source who has worked on US policy in Ukraine and knows Holmes.

Political specialist

As political counselor, his main job is to determine what is going on in Ukrainian politics.

Holmes often accompanied top US officials to meetings with Ukrainians as the note-taker in the last few years. It would not be unusual for him to accompany Sondland to these meetings, or for the two grab a meal in-between or after, the second source said.

Taylor laid out what he was told of the conversation between Sondland and Trump during his open testimony.

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

Taylor said that after the call, the staff member asked Sondland what Trump "thought about Ukraine."

"Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for," Taylor said adding he had learned about the overhead conversation on Friday and hadn't been aware of the conversation when he gave his deposition in October.

House Intelligence Chairman Adam Schiff, a California Democrat, asked Taylor if Sondland's statement meant that Trump cared more about the investigations "than he does about Ukraine?"

"Yes, sir," Taylor responded.

It remains unclear why this conversation, which draws Trump more directly to the center of the impeachment probe, is just now coming to the fore.

In his deposition, Holmes said Sondland later told him at the restaurant that Trump "doesn't give a s--t about Ukraine," and that his primary focus was on "big stuff that matters to him, like this Biden investigation that Giuliani is pushing."

During his public testimony before the House on Wednesday, Sondland was asked by Schiff if he disputes Holmes' testimony about the conversation. Sondland said he didn't dispute Holmes' recollection of the conversation.

A new revelation

Multiple sources, who have worked on Ukraine policy during the Trump administration, had not heard about the call that Holmes allegedly overheard, and only learned about it yesterday.

Sondland's conversation with Trump was "not good judgement" in that it was in a public place and over a cell phone, the second source said, but added that they were not surprised.

Sondland is known among his colleagues or referencing his conversations with the President, or his plans to call the president.

Sondland previously told lawmakers that he learned that Zelensky would have to make a public statement announcing investigations that would politically benefit Trump, in order to get the security assistance lifted.

He said "I do not specifically recall" how he learned of that link, in the amended portion of the testimony that he provided to congress.

Trump told reporters during a press conference with the Turkish President Erdogan last week that he did not know anything about the phone call with Sondland that Taylor referenced in his testimony.

According to the second source, Holmes never demonstrated any political leaning while they worked together.

"That is the thing about foreign service officers, they have their own views, but it's kind of irrelevant because it doesn't impact what they do," the second source said.

Holmes is the recipient of nine Department of State Superior Honor Awards. In 2014, he received an award for dissent as a mid-level foreign service officer, in which he urged a more strategic approach to South Asia policy.

At the time, an Office of the Special Representative for Afghanistan and Pakistan handled policy for the region as well as the State Department's Bureau of Central and South Asian Affairs.

Holmes wrote a memo detailing his opinion that the division "hindered our diplomatic effectiveness," according to AFSA. The memo then led the State Department to reconsider their approach to the region.