The whistleblower complaint related to President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE's discussions with a foreign leader filed by a member of the U.S. intelligence community involves Ukraine, according to The Washington Post.

Two sources familiar with the situation told the Post that the conversation that sparked a whistleblower complaint about an "alarming" promise made by Trump during a recent discussion with a foreign leader involved Ukraine.

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It wasn't reportedly clear whether Trump's conversation that sparked the complaint was specifically with Ukraine's president, but the Post reports that Trump did have a discussion with the newly-elected Ukrainian leader, Volodymyr Zelensky, just weeks before it was filed.

Democrats on the House Intelligence Committee say they were refused a briefing on the contents of the whistleblower complaint by Trump's acting director of national intelligence, Joseph Maguire, but received a briefing on the handling of the issue earlier Thursday.

House Intelligence Committee chairman Rep. Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) told The Hill on Thursday that Maguire had refused a congressional subpoena on the issue, and Democrats are now seeking legal remedy to compel release of the whistleblower report's contents.

“The [Intelligence Community Inspector General] determined that the complaint is both credible and urgent, which is why the Committee must move quickly. The Committee's position is clear — the Acting DNI can either provide the complaint as required under the law, or he will be required to come before the Committee to tell the public why he is not following the clear letter of the law, including whether the White House or the Attorney General are directing him to do so,” Schiff told The Hill.

“He has yet to provide the complaint in response to the Committee's subpoena, so I expect him to appear on Thursday, under subpoena if necessary,” Schiff added Thursday.

News of the probe's potential relation to Ukraine policy comes as The Independent reported that House lawmakers are investigating whether Trump pressed Ukraine's president during a July phone call to launch an investigation into former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, the Democratic 2020 primary front-runner, claiming that it would improve relations between the U.S. and Ukraine if Ukraine's government did so.