WASHINGTON – The House committees overseeing the impeachment inquiry of President Donald Trump are scheduled to hear from more witnesses on Tuesday and plan to release transcripts from two of the investigation’s most prominent witnesses.

The House Intelligence, Oversight and Foreign Affairs committees plan to release transcripts from testimony by Kurt Volker, the former U.S. special envoy for Ukraine, and Gordon Sondland, the U.S. ambassador to the European Union. Both are key witnesses in the investigation and were questioned about requests for political investigations and the delay of military aid to Ukraine. 

The committees also are expected to hear from two additional witnesses: Wells Griffith, a special assistant to the president and senior director for international energy and environment at the National Security Council, and Michael Duffey, associate director for national security programs at the White House Office of Management and Budget.

OMB Director Russ Vought has already said on Twitter that Duffey, who could hold information on the delay of military aid, won’t cooperate. It’s unclear whether Griffith will show up. He could provide information about Energy Secretary Rick Perry’s involvement in the controversy. 

Volker, Sondland testimony will be key 

Both Volker and Sondland spent nearly 10 hours behind closed doors fielding questions from lawmakers so the transcripts are likely to be lengthy and include a lot of details that have yet to become public. 

Both were included in text messages released by Democrats leading the investigation, which tied nearly $400 million in military funding and a White House meeting wanted by Ukrainians with investigations into Trump’s policial foes. They also showed unease by diplomats that crucial military aid was being conditioned on political investigations.

More: Trump told U.S. envoy that Ukraine was full of ‘terrible people’ who ‘tried to take me down’

More: A visual timeline of the text messages in the Trump-Ukraine affair

Volker, a career State Department official, said he realized that U.S.-Ukraine relations were in jeopardy because of Trump’s negative perception of the country, which he believed Giuliani was fueling. “I therefore faced a choice: do nothing, and allow this situation to fester; or try to fix it. I tried to fix it,” he told investigators. 

Volker worked with Giuliani and various White House officials to set up Trump’s July 25 call with Ukrainian president Volodymyr Zelensky and a potential White House visit. 

Sondland, a businessman and major Trump donor in 2016 before being named E.U. ambassador, sought to fend off the notion of a quid-pro-quo in the text messages by asking diplomats call him instead of writing back and pushing back. 

“The President has been clear no quid pro quo’s of any kind,” Sondland wrote in a September text message. “The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelensky promised during his campaign. I suggest we stop the back and forth by text.” 

More: Impeachment inquiry: Trump ambassador ‘disappointed’ with Rudy Giuliani’s influence in Ukraine policy

Sondland has come under scrutiny as additional witnesses have testified that they complained to him that it was not appropriate to link military aid and the investigations. Lawmakers questioned whether he was truthful during his deposition when he said: “nothing was ever raised to me about any concerns regarding our Ukrainian policy.” 

Sondland told lawmakers that withholding the aid for a political investigation would be wrong and denied being part of such a push, something that other witnesses have also disputed. 

“Let me state clearly: Inviting a foreign government to undertake investigations for the purpose of influencing an upcoming U.S. election would be wrong,” Sondland said. “Withholding foreign aid in order to pressure a foreign government to take such steps would be wrong. I did not and would not ever participate in such undertakings.”