House Democrats on Tuesday released the transcripts of testimony from U.S. Ambassador to the European Union Gordon Sondland and former special envoy to Ukraine Kurt Volker Kurt VolkerGOP senators request details on Hunter Biden's travel for probe Yovanovitch retires from State Department: reports Live coverage: Senators enter second day of questions in impeachment trial MORE as part of their impeachment inquiry.

Democrats are investigating whether Trump pressured Ukraine to open investigations 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 and his son Hunter Biden, and whether military aid was withheld from the country to convince Ukraine to open the probe. Volker helped set up the July 25 phone call between 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 and Ukrainian President Volodymyr Zelensky that is at the center of the inquiry.

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Sondland and Volker were involved in texts with another diplomat that have been publicly released. In the texts, Sondland said the president had been clear to him there had been no quid pro quo of any kind with Ukraine, before suggesting the men end the conversation over texts.

Sondland denied in his testimony that he pushed for Ukraine to do an investigation of Biden, but Democrats after hearing from other witnesses have questioned whether Sondland, a wealthy hotelier and GOP mega-donor before joining the administration, was truthful in his testimony.

Democrats have started the slow dribble of transcript releases in a week when a host of top administration officials are expected to defy congressional subpoenas and refuse to testify as part of the impeachment inquiry.

On Monday, four officials declined to appear in the Capitol for interviews, despite subpoenas seeking their participation. And two others were no-shows on Tuesday: Wells Griffith, a top energy expert at the National Security Council, and Michael Duffey, a senior national security aide at the White House Office of Management and Budget.

But the slow drip of transcript releases also ensures that the impeachment investigation stays near the top of the news cycle in a week when the House is on recess and requested witnesses are refusing to appear.