House Democrats have reportedly requested testimony from the top U.S. diplomat to Ukraine, who questioned 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 motivations for withholding aid to the country.

Democrats requested to depose diplomat William Taylor as part of their impeachment inquiry, but have not yet scheduled an interview with him or issued a subpoena, multiple sources familiar with the situation told CNN.

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In text messages turned over to the House during a deposition from 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, who served as the special envoy to Ukraine, Taylor suggested U.S. military aid was being withheld from Ukraine for political reasons.

Trump had delayed the hundreds of millions in dollars in aid, which had been approved by Congress, just days before he pressed Ukrainian President Volodymyr Zelensky on July 25 to open 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, a leading 2020 contender.

Taylor said in a Sept. 9 exchange that the delay of assistance had “shaken [the Ukrainians’] faith in us.”

“As I said on the phone, I think it’s crazy to withhold security assistance for help with a political campaign,” he added.

U.S. Ambassador to the the European Union Gordon Sondland, who had reportedly spoken with Trump on the issue, replied that Taylor was "incorrect about President Trump's intentions" and said he believed Trump had no intention of a "quid pro quo."

Sondland then suggested he and Taylor stop texting on the subject.

Democrats on House committees looking into Trump have also requested that other State Department officials including Sondland testify, but Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE has sought to block their testimony.

The House has subpoenaed Sondland and he said in a statement Friday that he plans to testify next week in light of that development. It is possible Pompeo will try again to prevent Taylor from testifying.

The Hill has reached out to the House committees and the State Department for comment.

Taylor is the chargé d’affaires of the embassy in Kiev. He served as ambassador to Ukraine from 2006 to 2009.