Congress’s impeachment investigation into US President Donald Trump turns on Tuesday to the US ambassador to the European Union and the role he may have played in trying to get Ukraine to probe Trump’s political rival Joseph Biden. Gordon Sondland, who donated $1 million to the Republican president’s inauguration committee, will meet behind closed doors with staff of three Democratic-led House committees. The impeachment probe is focusing on a whistleblower’s allegations that Trump leveraged nearly $400 million in aid to secure a promise from Ukraine’s president to investigate former vice president Biden and his son Hunter, who sat on the board of a Ukrainian energy company. The House of Representatives Foreign Affairs, Intelligence and Oversight Committees staff are expected to ask Sondland to explain why he became involved in dealings with Ukraine, which is not a member of the European Union. Sondland was a Seattle-based hotelier until Trump nominated him to his position as ambassador in May. He was confirmed by the US Senate in June and presented his credentials at the European Commission in July. According to text messages released by House committee leaders last week, Sondland was heavily involved in contacts with Ukrainian President Volodymyr Zelenskiy as he sought a meeting with Trump, and Ukrainian officials expressed concern at the administration’s decision to block nearly $400 million in US military assistance for Kiev. In one of the texts, for example, Sondland emphasized that Trump “really wants the deliverable.” Charges that Trump pressured Zelenskiy in a July 25 telephone call to investigate Biden, a leading rival in Trump’s 2020 re-election bid, while withholding the military aid, helped prompt House of Representatives Speaker Nancy Pelosi to announce a formal impeachment investigation last month. Trump has denied wrongdoing. Concerns about the call, and possible Trump threats to Ukraine, came to the attention of Congress in a report by a whistleblower. On Sunday, lawyers said a second whistleblower had come forward to substantiate that complaint.