WASHINGTON — A U.S. government employee overheard President Donald Trump ask Ambassador Gordon Sondland on the phone about "the investigations" and heard the ambassador reassure the president that the Ukrainians were "ready to move forward," a senior diplomat told Congress Wednesday in the first public impeachment hearing.

The revelation from William Taylor, the top U.S. diplomat in Ukraine, is a significant new development that emerged in a hearing that lawmakers had anticipated would largely reconstruct on television depositions that had taken place previously behind closed doors. But Taylor said he'd only learned from the staffer about the call last week — after he'd already been deposed privately.

Taylor, who served as the acting ambassador in Ukraine, said his staffer had informed him of events that transpired on July 26, while several senior Trump administration officials were visiting Ukraine. He said one of his staffers had accompanied Sondland to a meeting with Andriy Yermak, a top aide to Ukrainian President Volodymyr Zelenskiy.

After that meeting, Taylor said, his staffer was at a restaurant with Sondland, the U.S. ambassador to the European Union, and overheard him on the phone with Trump. Taylor testified that his staffer heard Trump ask Sondland about "the investigations."

"Ambassador Sondland told President Trump that the Ukrainians were ready to move forward," Taylor testified in his opening statement to the House Intelligence Committee.

Taylor said that after the call, his staffer asked Sondland about Trump's views of Ukraine.

Relaying what the staffer had told him about the conversation, Taylor said, "Ambassador Sondland responded that President Trump cares more about the investigations of Biden, which Giuliani was pressing for."

Trump has been accused of pressuring Ukraine to investigate former Vice President Joe Biden and his son Hunter's work for a Ukrainian energy firm. Taylor said Rudy Giuliani, the president's personal attorney, directed an "irregular" back channel on Ukraine policy at odds with well-established U.S. policies.

Taylor did not identify the aide who informed him about the incident, but two sources familiar with the matter told NBC News that it was David Holmes, the State Department official just added to the calendar to testify in closed session Friday.

Holmes is the counselor for political affairs at the U.S. Embassy in Ukraine. As Wednesday's hearing got underway, two officials working on the impeachment inquiry told NBC News that Holmes' deposition had been scheduled.

Pressed for more details about the incident by the committee's chairman, Adam Schiff, D-Calif., Taylor said Sondland had told his staffer that Trump also cares about Burisma, the Ukrainian natural gas company associated with Hunter Biden that Trump wanted the Ukrainians to investigate.

Schiff asked Taylor whether "the investigations" referred to probes into the Bidens and a conspiracy related to the 2016 election.

"That is correct," Taylor said.

Taylor said the overheard conversation between Trump and Sondland had taken place on a cellphone while Sondland was visiting the Ukrainian capital.

Sondland, through his attorney, declined to comment.

It was unclear whether Sondland's remark to Trump that the Ukrainians were ready to proceed with investigations was based on his conversation with Yermak minutes earlier. But the July 26 incident marks a second time that investigations came up in relation to a meeting between Sondland and Yermak, the Ukrainian presidential aid.

Sondland, in a sworn declaration to Congress, acknowledged having discussed investigations with Yermak in another meeting in Warsaw, Poland, on Sept. 1. He said he'd told Yermak that aid to Ukraine would "likely" not be restored until Ukraine delivered a statement that they had been discussing committing to corruption investigations.

Although Sondland had not mentioned that meeting in his private deposition by the House, he later told Congress in writing that his memory had been refreshed after other witnesses testified about the chain of events.

As the top diplomat in Ukraine, Taylor oversees a large staff at the U.S. Embassy in Kyiv. It's not unusual for diplomats from the embassy to accompany visiting officials to meetings with members of a foreign government.

The revelation is likely to spark heavy interest from congressional investigators as they continue to piece together the timeline of the Trump administration's pressure campaign on Ukraine, even as the impeachment proceedings move into the public hearing stage.

Taylor says that since his staffer didn't inform him about the incident until Friday, he wasn't aware of it on Oct. 22, when he testified behind closed doors. The House later released a transcript of his lengthy deposition.

Taylor said that once he learned of the information, he reported it through his attorney to State Department lawyers, as well as to both the Democratic and Republican leadership of the House Intelligence Committee.

"It is my understanding that the committee is following up on this matter," Taylor said.

Holmes, meanwhile, once won an award for voicing dissent within the government when he saw something amiss.

Holmes has a history of speaking up when he disagrees, according to an NBC News review of archived materials from the American Foreign Service Association, or AFSA, the union that represents U.S. diplomats. In 2014, he won the AFSA's William R. Rivkin Award for Constructive Dissent, which honors a mid-career foreign service officer for intellectual courage in speaking up.

At the time of the award, Holmes was senior energy officer at the U.S. Embassy in Moscow. He was recognized for his work on Afghanistan and South Asia, during which he filed a formal dissent channel message in February 2013. He argued that the division of authority for Afghanistan-Pakistan policy among different parts of the State Department "hindered our diplomatic effectiveness."

"My efforts over this period, and then my formal dissent, were intended to give a voice to an important perspective that I felt lacked an advocate," Holmes was quoted as saying in an article in the September 2014 edition of The Foreign Service Journal, the union's monthly magazine.

A dissent cable is a unique State Department mechanism that lets diplomats voice disagreement about U.S. policies, with protections against retribution. They've been used previously during the Vietnam War and in 2017, when diplomats objected to Trump's travel ban on people from several Muslim-majority nations.

The article says Holmes joined the foreign service in 2002 and has served in Afghanistan and India, as well as on the White House National Security Council as Afghanistan director from 2011 to 2012, during the Obama administration. The article includes an official White House photo of Holmes in the Oval Office shaking hands with President Barack Obama.

A comparison of the photos of Holmes in the AFSA article and on Holmes' Facebook page confirms that the David Holmes who won the award in 2014 is the same David Holmes who is now posted to the U.S. Embassy in Kyiv.

Holmes did not immediately respond to a request for comment, nor did a representative for the AFSA. But two former U.S. officials described him as a straight-shooter.

"Struck me as very competent and a rising star," said a former U.S. ambassador who has worked with Holmes. A former White House official who overlapped there with Holmes said he "kept his nose to the grindstone and worked a complex issue effectively."