U.S. 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 Volodymyr Zelensky an audience with President Donald Trump.

Then-National Security Adviser John Bolton had been noncommittal about scheduling a meeting between Trump and new Ukrainian President Volodymyr Zelensky but Sondland directly contradicted Bolton by telling the Ukrainians that in fact, Trump was committed to meeting with Zelensky 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 Zelensky 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.

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The account also appears to undercut Sondland's text message to a senior U.S. official in Ukraine on Sep 9 that Trump "has been crystal clear no quid pro quo's (sic) of any kind" with the Ukrainian government.

The initial July 10 meeting in the West Wing included senior Zelensky adviser Andriy Yermak and Oleksandr Danyliuk, who was the incoming Ukrainian national security secretary.

Bolton, Sondland, Energy Secretary Rick Perry, Hill and former Ukraine envoy Kurt Volker also attended.

Ukraine is not a member of the EU, which raises serious questions about why U.S. Ambassador to the EU 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.

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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.