US President Donald Trump said on Thursday that he would deliver on his promise to release the transcript of an April call with Ukrainian President Volodymyr Zelenskiy.

The transcript of a July 25 call Trump made with the Ukrainian president in which he asked him to investigate the son of his political rival Joe Biden has become the subject of a Democratic-led impeachment inquiry into the president.

Trump’s critics are accusing him of withholding almost $400 million in security aid to Ukraine as leverage to pressure Zelenskiy to publicly declare an investigation.

Earlier in the month, President Trump opposed impeachment enquiry, saying that there should be no public hearings during the House of Representatives’ impeachment enquiry against him, and directed White House Chief of Staff Mick Mulvaney not to appear before the committees investigating Trump’s phone call to Ukraine.

Trump administration had ordered officials not to participate in the House enquiry. But lawmakers have spent weeks hearing from current and former government witnesses, largely from the State Department, as one official after another has relayed his or her understanding of events.

Last month, US former national security advisor was so alarmed by a White House–linked effort to pressure Ukraine to investigate Democrats, he told aide Fiona Hill to alert the National Security Council’s chief lawyer, Hill told House impeachment investigators in her 10-hour deposition.

Late September, the impeachment inquiry, which House Speaker Nancy Pelosi initiated over a complaint by an anonymous whistleblower, is looking into White House’s alleged efforts to withhold military aid to have Ukraine investigate a Trump’s political rival, Joe Biden.