Acting US Ambassador to Ukraine Bill Taylor arrives for a deposition in the House Impeachment inquiry on Capitol Hill in Washington, DC, October 22, 2019.Saul Loeb | AFP | Getty ImagesA top diplomat testified Tuesday in the impeachment inquiry into President Donald Trump after House Democrats issued a subpoena to him following State Department attempts to block his appearance, an official working on the probe told NBC News.Acting U.S. Ambassador to Ukraine Bill Taylor appeared Tuesday morning on Capitol Hill for a scheduled deposition in the House impeachment investigation. His text messages with other administration officials — shared with lawmakers by former U.S. special envoy to Ukraine Kurt Volker as part of the inquiry — have helped fuel suspicions that the Trump administration was trying to arrange a politically motivated quid pro quo with Ukraine.”In light of an attempt by the State Department to direct Ambassador William Taylor not to appear for his scheduled deposition, and efforts by the State Department to also limit any testimony that does occur, the House Intelligence Committee issued a subpoena to compel his testimony this morning,” an official working on the impeachment probe told NBC on Tuesday morning.”As is required of him, Ambassador Taylor is now complying with the subpoena and answering questions from both Democratic and Republican Members and staff,” the official said.The probe stems from a whistleblower complaint that raised alarms about Trump’s request to Ukraine’s president in July to “look into” former Vice President Joe Biden and his son, as well as Ukraine’s alleged role in efforts to interfere in the 2016 presidential election.The White House has vowed not to cooperate with the impeachment inquiry, which was announced by House Speaker Nancy Pelosi, D-Calif., and is being led by the House Intelligence, Foreign Affairs and Oversight committees.Secretary of State Mike Pompeo has also decried the impeachment proceedings. He has accused Democrats of trying to “bully” State Department officials.Taylor’s texts with the U.S. ambassador to the European Union, Gordon Sondland, have made him a significant figure in the probe.In one Sept. 9 exchange, Taylor told Sondland: “I think it’s crazy to withhold security assistance for help with a political campaign.”Sondland responded: “Bill, I believe you are incorrect about President Trump’s intentions. The President has been crystal clear no quid pro quo’s of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign.”Click here to read those texts.Sondland testified behind closed doors last week, defying an attempt by the State Department to block him from doing so. Taylor left Ukraine for Washington, D.C., last week, after Democrats requested his testimony.Sondland reportedly testified that Trump “directed” his personal lawyer, former New York Mayor Rudy Giuliani, to participate in efforts to get Ukraine to investigate the Bidens, as well as a conspiracy theory that Ukraine played a role in attempts to undermine Trump’s candidacy in 2016.