WASHINGTON — Lawmakers plan to grill Ambassador to the European Union Gordon Sondland on Thursday about a private discussion he had with top Ukrainian officials in the White House in which he explicitly mentioned the Ukrainian gas company linked to Hunter Biden, amid negotiations over granting Ukraine’s new president an audience with President Donald Trump, NBC News has learned.

Sondland’s meeting with the Ukrainians just steps away from the White House Situation Room came minutes after a larger West Wing meeting that included then-National Security Adviser John Bolton, who had been noncommittal about scheduling a meeting between Trump and new Ukrainian President Volodymyr Zelenskiy. Sondland directly contradicted Bolton by telling the Ukrainians that in fact, Trump was committed to meeting with Zelenskiy on the condition he open a corruption investigation, two people told about the matter tell NBC News.

Bolton abruptly ended the meeting.

But, the individuals say, Sondland then invited the Ukrainian officials to continue the conversation separately, escorting them to a private room in the White House basement, the individuals said. That’s when Sondland was overheard discussing Burisma Holdings, whose board of directors former Vice President Joe Biden’s son Hunter Biden joined in 2014.

Sondland’s explicit conditioning of a meeting between Zelenskiy and Trump prompted Bolton to direct then-Trump Russia adviser Fiona Hill to report the situation to White House lawyers. Although some details about the meeting had been previously disclosed, this account provides the most extensive chronology to date of the chain of events that led Bolton to call the shadow Ukraine policy aimed at advancing Trump’s political interests “a drug deal,” as NBC News has reported.

The account also appears to undercut Sondland’s text message to a senior U.S. official in Ukraine on Sept. 9 that Trump “has been crystal clear no quid pro quo's (sic) of any kind” with the Ukrainian government. The fact that Hunter Biden had joined Burisma’s board had been widely reported for years and discussed publicly by Trump personal lawyer Rudy Giuliani. But Sondland plans to tell Congress he was unaware of Hunter Biden’s ties to the company at the time, a person with knowledge of his testimony says, an assertion that lawmakers are expected to treat with extreme skepticism.

Bolton had no comment. The White House did not immediately respond to requests for comment. Hill, through her attorney Lee Wolosky, declined to comment.

Sondland declined to comment.

But in his testimony on Thursday, Sondland plans to say he kept Hill updated on all his activity on Ukraine through extensive emails that the State Department has refused to give to Congress, a person familiar with Sondland’s testimony tells NBC News. He plans to say he’s “frustrated” that the State Department won’t produce the documents because he says they corroborate the testimony he plans to give, the person said.

Secretary of Energy Rick Perry, from right, chats with U.S Ambassador to the European Union, Gordon Sondland at the 1st EU-U.S. Energy Council High-Level B2B Forum on LNG at the European Commission in Brussels, Belgium on May 2, 2019. Dursun Aydemir / Anadolu Agency/Getty Images file

The initial July 10 meeting in the West Wing included senior Zelenskiy adviser Andrey Yermak and Oleksandr Danyluk, who was the incoming Ukrainian national security secretary. Bolton, Sondland, Energy Secretary Rick Perry, Hill and former Ukraine envoy Kurt Volker also attended.

In the meeting, Bolton told the Ukrainian officials that the White House didn’t want to commit to a meeting ahead of parliamentary elections in Ukraine scheduled for the following month, because it was unclear whether Zelenskiy would have a governing majority, the individuals familiar with the situation said.

But Sondland interjected, the individuals said, saying he already had a commitment for Trump to meet with Zelenskiy — if the new Ukrainian president would launch the investigation Trump had been seeking.

As Bolton ended the meeting, Sondland suggested they relocate to “discuss next steps,” bringing the visiting Ukrainians to the Ward Room, in the White House basement, the two individuals said.

Bolton pulled aside Hill, the Europe expert, and asked her to follow them into the meeting to learn what they were discussing, the individuals said.

The individuals said Hill walked into the Ward Room as Perry was leaving, and heard the officials discussing conditions for a meeting between Trump and Zelenskiy, including a corruption investigation. The individuals said that Hill heard Burisma Holdings specifically mentioned.

After Hill told the group she thought the discussion was inappropriate, the meeting ended. At Bolton’s direction, Hill then reported the situation involving the Ukraine discussions to John Eisenberg, the top National Security Council lawyer, NBC News has previously reported.

“It seems highly unusual for a State Department official whose portfolio doesn’t even naturally include Ukraine to be convening what seems to be a furtive gathering of officials in the West Wing without another White House official present,” said Ned Price, a former White House National Security Council official in the Obama administration and an NBC News national security analyst. “This is just another data point that Ambassador Sondland wasn’t doing the work of the country, he was doing Donald Trump’s political bidding.”

Perry, who has also been subpoenaed in the impeachment inquiry, has emerged alongside Volker and Sondland as part of a troika of officials working outside the normal National Security Council process to drive U.S. policy on Ukraine, just as Trump and Giuliani were seeking to get the Ukrainians to investigate Trump’s political rivals.

Late Wednesday, Perry said he’d called Giuliani this spring at Trump’s direction to facilitate a Trump-Zelenskiy meeting. In comments first reported by The Wall Street Journal, Perry said that Giuliani had brought up allegations of Ukrainian interference in the 2016 U.S. election – which are unfounded – and told him to “be careful” about Zelenskiy.

Giuliani confirmed the call with Perry, telling NBC News: “Everything I said there I probably said on television 50 times.”

Volker, who resigned from his post last month as U.S. special representative for Ukraine negotiations, did not respond to multiple requests for comment from NBC News. A spokeswoman for Perry did not immediately respond to a request for comment.

Earlier this month, Volker testified before the House for more than eight hours behind closed doors. Nothing in his testimony or in documents he handed over to Congress contradicted this chronology, according to a person with knowledge of his testimony.

Sondland, a major donor to Trump’s inaugural committee who the president named as his EU ambassador, is expected to be questioned at length by lawmakers about the July 10 meeting and his other efforts on Ukraine during his highly anticipated deposition Thursday.

Ukraine is not a member of the EU, raising serious questions about why Sondland was involved in Ukraine policy in the first place, although he told a Ukrainian television network in an interview that Trump had given him the “special assignment” of overseeing Ukraine relations.

Other current and former officials have offered testimony critical of Sondland — including Hill, who testified that Sondland and Giuliani circumvented her and the National Security Council to run a shadow foreign policy on Ukraine.

Hill also told lawmakers that she considered it a direct counterintelligence threat to the United States, people familiar with her testimony told NBC News.

The State Department has already missed a deadline to turn over documents that the House has subpoenaed, including the ones that Sondland wants released as he works to clear his name.

The State Department did not immediately respond to a request for comment. But Secretary of State Mike Pompeo has said his agency will “obviously do all the things we’re required to do by law.”

“Sadly, there have been congressional inquiries that have harassed and abused State Department employees by contacting them directly and seeking to have them provide documents,” Pompeo said earlier this month in Athens, criticizing House Democrats for circumventing State Department lawyers in their requests. “That’s harassment, and I’ll never let that happen to my team.”