Gordon Sondland, the U.S. ambassador to the European Union who has become a key figure in the House Democrats’ impeachment inquiry, will appear for questioning in the Capitol on Thursday, he announced.

Sondland said he would honor a House subpoena compelling him to appear and answer questions about President Trump’s efforts to get Ukraine to open an investigation into former Vice President Joe Biden as well as actions by Democrats to solicit help from Ukraine to discredit Trump’s 2016 campaign.

Sondland was scheduled for a closed-door deposition on Monday but did not show up after the State Department prohibited his testimony.

Three House committees led by Democrats are conducting the impeachment investigation.

Sondland is a major GOP donor who Democrats believe has information about whether Trump delayed $400 million in security aid to Ukraine in an effort to get the Ukrainian government to investigate Biden and the Democrats.

Sondland’s lawyers said Friday that while the ambassador will appear to answer questions, he will not bring the documents requested by Democrats because the documents are under the control of the State Department.

Democrats want to see Sondland’s texts and emails relating to Ukraine’s security aid.

Marie Yovanovitch, the former U.S. ambassador to Ukraine who was recalled from the post early, is scheduled to appear before the committee on Friday.