WASHINGTON (CN) – Lawmakers spearheading President Donald Trump’s impeachment inquiry released a trove of text messages late Thursday they say outline the administration’s pressure campaign on Ukraine to investigate political opponents of Trump in exchange for U.S. military aid.

The messages, according to a letter from the Democratic chairmen of the House Intelligence, Foreign Affairs and Oversight committees, are among the clearest pieces of evidence yet showing State Department officials – at Trump’s direction – were willing to exchange a meeting between Ukraine’s President Volodomyr Zelensky and Trump for Kiev’s willingness to investigate former Vice President Joe Biden.

Spanning from July 19 to Sept. 9, the messages were first provided to Congress Thursday by Kurt Volker, the former special envoy to Ukraine who resigned last week.

Thursday’s letter from the chairmen says the text messages made it abundantly clear why Secretary of State Mike Pompeo wished to block testimony from ambassadors implicated in the scandal like Volker, who testified behind closed doors for several hours Thursday.

The texts are exchanges between Volker; William Taylor, the U.S. chargé d’affaires in Kiev; Gordon Sondland, the ambassador to the European Union; Ukrainian adviser Andrey Yermak; and President Trump’s personal attorney, Rudy Giuliani.

“Heard from the White House – assuming President Z convinces Trump he will investigate/ “get to the bottom of what happened” in 2016, we will nail down a date for visit to Washington,” Volker wrote to Ukraine adviser Yermak on July 25, the morning of Trump’s call with Zelensky.

The July 25 call prompted a U.S. intelligence official to submit the whistleblower complaint that ultimately triggered the president’s impeachment inquiry.

Throughout the texts, Volker and Sondland emphasized Ukraine should be unequivocal in its support of an investigation into Joe Biden and Burisma Holdings, the energy company that had the former vice president’s son, Hunter Biden, sitting on its board of directors.

One text message from August revealed Volker proposing to Sondland that the U.S. give Zelensky a statement to read during a news conference.

“Special attention should be paid to the problem of interference in the political processes of the United States especially with the alleged involvement of some Ukrainian politicians,” Volker wrote on August 13. “We intend to initiate and complete a transparent and unbiased investigation of all available facts and episodes including those involving Burisma and the 2016 elections.”

Other texts, including those from U.S. chargé d’affaires in Ukraine, William Taylor, illuminate pressure that officials felt after the U.S. withheld security assistance to Ukraine.

“The message to the Ukrainians (and Russians) we send with the decision on security assistance is key,” Taylor wrote in a Sept. 9 message to Sondland. “We have already shaken their faith in us. Thus my nightmare scenario.”

In another text Taylor said he thought it was “crazy to withhold security assistance for help with a presidential campaign.”

Sondland responded by saying he believed Taylor was incorrect about Trump’s intentions and insisted there was no quid pro quo “of any kind.” He then suggested they cease discussing the subject over text.

President Trump froze roughly $400 million in U.S. military aid to Ukraine shortly before his July 25 call with Zelensky, though he and other administration officials have steadfastly denied the freeze was retaliatory.

The texts were released the same day Trump told reporters gathered outside of the White House that he supported foreign countries conducting investigations into his political opponents.

“If they were honest about it, they would start a major investigation into the Bidens. … They should investigate the Bidens,” Trump said, fielding direct questions about what he wanted Zelensky to do. “Likewise, China should start an investigation into the Bidens because what happened in China is just about as bad as what happened with Ukraine.”

Trump said he had not asked China’s President Xi Jinping to launch a probe just yet but that it was “certainly something we should start thinking about.”

Republican lawmakers have continued with their staunch support of the president since House Speaker Nancy Pelosi formally announced the start of an impeachment inquiry. They have dubbed the investigation just another attempt by Democrats to remove Trump from office without cause and engage in partisan politics.

House Intelligence Committee Chairman Adam Schiff, House Foreign Affairs Committee Chairman Eliot Engel and House Oversight Committee chairman Elijah Cummings — all of whom are tasked with compiling evidence for the investigation — see things quite differently.

“This is not normal or acceptable,” the chairmen wrote. “It is unethical, unpatriotic and wrong. American Presidents should never press foreign powers to target their domestic political rivals.”

On Friday, the intelligence community’s inspector general, Michael Atkinson, will testify privately before the House Intelligence Committee. Atkinson was the first to receive – and find credible – the whistleblower complaint that spurred the impeachment inquiry.

Ahead of Atkinson’s testimony, Ukraine’s prosecutor general Ruslan Ryaboshapka said his office will soon begin review of several cases shuttered by his predecessors, including some related to Mykola Zlochevsky, owner of Bursima Holdings.

“We are now reviewing all the cases that were closed or split into several parts or were investigated before, in order to be able to rule to reverse those cases where illegal procedural steps were taken,” Ryaboshapka said before adding that nary one Ukrainian official or politician has attempted to influence his decisions regarding any criminal case.