WASHINGTON (Reuters) – Testimony by two of the “three amigos” charged with running Ukraine policy for President Donald Trump will be made public on Tuesday, as Democratic-led congressional committees release more transcripts from their impeachment investigation. Representative Adam Schiff, chairman of the House of Representatives Intelligence Committee, told reporters on Monday that transcripts would be released of interviews with Kurt Volker, Trump’s former special representative for Ukraine negotiations, and Gordon Sondland, the U.S. ambassador to the European Union. Witnesses have testified that Volker and Sondland, with Trump’s secretary of energy, Rick Perry, were known as the “three amigos,” responsible for Trump’s unofficial channel to Ukrainian government officials. Volker resigned as special representative in September. He testified to the House Foreign Affairs, Intelligence and Oversight panels for more than eight hours behind closed doors on Oct. 3. Sondland, a major Trump donor, testified on Oct. 17. Perry, a former Texas governor who said he was resigning from his Cabinet post as of Dec. 1, has refused to testify so far. Tuesday’s will be the second public release of testimony in the impeachment investigation of Trump that Democratic House Speaker Nancy Pelosi formally launched on Sept. 24. On Monday, the committees released transcripts of testimony by Marie Yovanovitch, whom Trump abruptly recalled as ambassador to Ukraine in May, and Michael McKinley, a former top adviser to Secretary of State Mike Pompeo. In their interviews, Yovanovitch and McKinley said the State Department was being used for domestic political purposes under Trump and warned that would hurt American interests. The House investigation is focused on a July 25 phone call in which Trump asked Ukrainian President Volodymyr Zelenskiy to investigate former Vice President Joe Biden, a leading Democratic rival as Trump seeks re-election in November 2020. Related CoverageFour ways Trumpâ€™s tax returns could come to lightTrump froze nearly $400 million in U.S. military assistance to Ukraine shortly before speaking to Zelenskiy, prompting accusations from Democrats that he had misused U.S. foreign policy for personal gain. Strongly backed by his fellow Republicans in Congress, Trump has denied wrongdoing and accused Democrats of unfairly targeting him in the hope of reversing his surprise victory in the 2016 presidential election. PREPARING FOR PUBLIC HEARINGS The committees began releasing interview transcripts as they prepare for public hearings that could start this month. If the House eventually votes to impeach Trump, a trial in the Republican-controlled Senate would be held. Trump would not be removed from office unless two-thirds of the senators who are present vote to convict him – an unlikely prospect at this point. According to accounts of their testimony already public, both Sondland and Volker discussed communications between Trump’s private attorney, Rudy Giuliani, and Ukrainian officials with House investigators. Volker turned over text messages concerning Ukraine among himself, Giuliani and other diplomats. Sondland testified that Trump directed him, Perry and Volker to talk to Giuliani about his concerns about corruption in Ukraine and that the three were dismayed by that order. He said he did not understand “until much later” that Giuliani’s agenda included a push for Ukraine to investigate Biden. Sondland also said it would be wrong to get a foreign government to launch investigations in order to influence a U.S. election. The Trump administration has directed U.S. officials not to cooperate with the probe. Two administration officials who had been scheduled to appear for closed-door testimony did not show up on Tuesday: Michael Duffey, associate director of the White House budget office, and Wells Griffith, senior director for international energy and the environment on the White House National Security Council. Four White House witnesses also failed to testify on Monday. Schiff said on Monday that could potentially lead to obstruction of Congress charges against the president.