WASHINGTON – At the behest of President Trump and his personal lawyer, U.S. diplomats engaged in a frenetic, months-long effort to push Ukraine's newly elected president to publicly promise he would order an investigation into Joe Biden's son and also probe a conspiracy theory about Ukraine's alleged role in the 2016 U.S. election.

In exchange, the diplomats believed, Trump would reward Ukrainian President Volodymyr Zelensky with a highly sought-after meeting with Trump at the White House and the release of nearly $400 million in U.S. military aid that Trump had put on hold.

Text messages between Kurt Volker, then-Trump's special envoy for Ukraine, and other U.S. diplomats – released late Thursday by House Democrats leading an impeachment inquiry – paint a picture of American foreign policy decisions being driven by Trump's domestic political concerns and an ad-hoc agenda crafted by Rudy Giuliani, Trump's private attorney.

"Heard from the White House," Volker wrote in a text to a top Zelensky adviser on July 25, just before Trump and Zelensky were scheduled to speak by phone in a call that helped spark the impeachment inquiry.

"Assuming President Z (Zelensky) convinces trump he will investigate/"get to the bottom of what happened" in 2016, we will nail down a date for visit to Washington. Good luck!," Volker told his Ukrainian counterpart.

House Democrats released the text messages after Volker, who resigned from the Ukraine envoy post last week, spent more than nine hours Thursday testifying behind closed-doors as part of the Democrats' impeachment inquiry.

"These text messages reflect serious concerns raised by a State Department official about the detrimental effects of withholding critical military assistance from Ukraine, and the importance of setting up a meeting between President Trump and the Ukrainian President," the chairmen of the three House committees leading the impeachment probe said in a statement released Thursday night. The three chairmen are: Intelligence Committee Chair Adam B. Schiff, Oversight and Reform Committee Chair Elijah E. Cummings, and Foreign Affairs Committee Chair Eliot L. Engel.

The Democrats said the messages show that State Department employees were deeply concerned that U.S. military assistance and a Trump-Zelensky meeting "were being withheld in order to place additional pressure on Ukraine to deliver on the President’s demand for Ukraine to launch politically motivated investigations."

Republicans characterized the proceedings as a sham that did little to provide Democrats any ammunition to impeach Trump.

Speaking to reporters outside the hearing room as Volker was still testifying, Rep. Jim Jordan, an Ohio Republican, slammed Schiff, D-Calif., for not allowing State Department lawyers to participate in the session. He also said the testimony he heard from Volker did not support Democrats' impeachment narrative.

"Ambassador Volker has been very impressive and has said nothing that coincides with what the Democrats are seeing with their whole impeachment narrative," said Jordan, the top Republican on the House Committee on Oversight and Reform.

But the text messages demonstrate a high level of apprehension among State Department officials that Trump had linked U.S. assistance to Ukraine with his demands that Zelensky investigate the 2016 election and Burisma, a Ukrainian gas company that employed Biden's son, Hunter.

Bursima's owner had come under scrutiny by Ukraine's top prosecutor, but there were no charges brought and Ukrainian officials have said there's no evidence of wrongdoing by Hunter Biden or his father.

"Most impt is for Zelensky to say that he will help with investigation," Volker wrote in a July 17 message to two other U.S. diplomats with purview over Ukraine policy: Gordon Sondland, the ambassador to the European Union, and Bill Taylor, the top American diplomat stationed in Ukraine.

After Trump and Zelensky spoke on July 25, Zelensky's top aide, Andriy Yermak texted Volker to say the call "went well." He added: "Please remind Mr. Mayor to share the Madrid dates." That was a reference to Giuliani's plans to meet with Zelensky's advisers, in Madrid in August, to follow up on the Trump-Zelensky phone call.

For months, Giuliani had been pressing the Ukrainians for damaging information on the Bidens. The Ukrainians seemed to understand the linkage Giuliani and others were making. Zelensky, a political neophyte elected in April, was eager to a public demonstration of U.S. support, particularly as his country faced sustained Russian aggression.

Zelensky's aide, Yermak, seemed to agree in August that the new president would publicly announce an investigation into Bursima, once the White House had set a date for a Zelensky-Trump meeting in Washington.

“Once we have a date, will call for a press briefing, announcing upcoming visit and outlining vision for the reboot of US-UKRAINE relationship, including among other things Burisma and election meddling in investigations,” Yermak texted Volker on Aug. 10.

But the Trump administration continued to delay a meeting announcement.

And Taylor, the lead diplomat in Kiev, seemed alarmed by the apparent connection between Trump's freeze on U.S. aid to Ukraine and the president's demands for probes into Biden and the 2016 presidential election.

"Are we now saying that security assistance and WH meeting are conditioned on investigations?" Taylor asked in a Sept. 1 text message to Volker and Sondland. "Call me," Sondland texted back.

Taylor later raised a "nightmare" scenario, in which Zelensky would promise to order the investigations that Trump wanted, but the U.S. would still not release the military assistance, which Ukraine needed to counter Russian aggression.

"The Russians love it," Taylor texted on Sept. 8. The next day, he texted again that the U.S. had already "shaken (Ukraine's) faith in us" and warned Sondland: "Counting on you to be right about this."

Sondland suggested he wasn't sure what would happen. "Let's hope it works," he texted to Taylor on Sept. 9.

Taylor responded: "As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign."

Sondland pushed back, saying Trump had been "crystal clear no quid pro quo's of any kind." He told Taylor to call "S," presumably referring to Secretary of State Mike Pompeo, if he wanted to discuss the matter further.

Zelensky never made a public commitment to launch any U.S.-related investigations. Trump eventually released the military aid after a bipartisan backlash from Congress.

Trump ordered the hold on the aid to Ukraine in mid-July, shortly before he spoke with Zelensky. He has given contradictory explanations for that decision, initially saying he wanted to be sure Ukraine would crack down on corruption and later suggesting he wanted European allies to contribute more to Ukraine's defense.

Democrats emerged from Thursday's session convinced the aid was used as part of an attempted quid-pro-quo.

"It was further evidence of the underlying fact that has now spurred this formal impeachment inquiry," Rep. Gerry Connolly, D-Va., told USA TODAY Thursday evening. The president "used military aid and other leverage to attempt to extort (the Ukrainian leader) for a narrow partisan domestic political reason: getting dirt on his prospective political opponent."

A second Democrat, Rep. Eric Swalwell of California, said Volker's testimony demonstrated that Trump held out the prospect of a White House meeting with Zelensky on the condition that Ukraine investigate Biden – as well as Ukraine’s alleged involvement in interference with the 2016 U.S. election. The American intelligence community has concluded that Russia tried to sway the 2016 election in Trump's favor, a fact that has clouded Trump’s presidency.

"For Zelensky to get a meeting with Trump, Zelensky had to, one, investigate the 2016 election, essentially go back and exonerate the Russians’ role," Swalwell told reporters after Volker's deposition ended. "And two, that Zelensky would have to investigate Biden. That was an understood predicate for the meeting."

Rep. Lee Zeldin, a New York Republican, suggested that Democrats had cherry-picked from Volker's texts to provide a misleading account of his testimony.

"Instead of House Dems releasing Ambassador Volker's texts drip by drip, just release all of his texts in Congress' possession right now as well as today's transcribed interview," Zeldin tweeted Thursday night. "Just give the media & public EVERYTHING so Americans can form their own independent conclusions."

More:Here are 5 questions about the Trump-Ukraine controversy we still don't have answers to

Volker, a former foreign service officer and longtime Europe expert, is the first official interviewed by House Democrats as they investigate potentially impeachable allegations that Trump used the power of his office to seek foreign interference in the 2020 presidential election. Thursday's deposition, led by staff lawyers with the House Intelligence Committee, was attended by a handful of lawmakers from both sides.

The Democrat-led House launched an impeachment inquiry last week to examine the president's pressure campaign, which became public when the White House released a summary of a July 25 phone call between Trump and Zelensky.

More:Impeachment pressure: Trump says China should investigate Joe Biden, family

Volker played a central role in connecting Giuliani with Ukrainian officials – a step critics say is highly inappropriate. And he was named in the explosive whistleblower complaint that sparked the impeachment proceedings.

Volker resigned less than 24 hours after Giuliani posted a private text message from the special envoy – in which Volker offered to set up a meeting with a top adviser to Zelensky. Giuliani was trying to get Ukrainian officials to investigate Biden, the former vice president who is seeking to unseat Trump in 2020.

According to the whistleblower complaint, Volker and Sondland had met with Giuliani to try to "contain the damage" his efforts were having on U.S. national security. The whistleblower said Volker and Sondland also met with Ukrainian officials to help them navigate the "differing messages" they were getting through official U.S. government channels and Giuliani's private outreach.

Volker agreed to Thursday's deposition, even as Secretary of State Mike Pompeo has delayed other State Department officials from testifying.

The complaint:Read the full declassified text of the Trump whistleblower complaint

Volker was named as Trump's special envoy in July 2017, by then-Secretary of State Rex Tillerson. He came to the post with a stellar resume: a former U.S. ambassador to NATO in the George W. Bush administration and former adviser to Sen. John McCain, R-Ariz., a longtime Russia critic.

He took the envoy job on a volunteer basis, while continuing to serve as executive director of the McCain Institute for International Leadership at Arizona State University.

More:Democrats see echoes of Russia in Trump’s call for China to investigate Joe Biden