The explosive texts, released this week by House Democrats, were obtained as part of the ongoing impeachment inquiry of President Donald Trump in Congress.
The chats include Kurt Volker, the U.S special envoy to Ukraine at the time; Bill Taylor, the top U.S. diplomat to Ukraine; and Gordon Sondland, the United States ambassador to the European Union.
Volker also exchanges messages with Andrey Yermak, a top aide to Ukrainian President Volodymyr Zelenskiy.
Here are five things to know:
The quid pro quo
Democrats have yet to release the full transcripts of all the texts. But so far, the most revealing seem to come from Volker, Trump’s special envoy to Ukraine who testified behind closed doors this week.
In the exchanges with other diplomats and with Ukraine’s Yermak, Volker makes clear that the U.S. won’t agree to a summit between the two leaders unless Ukraine announces plans for an investigation.
“Heard from the White House,” he tells Yermak shortly before a July 25 phone call between Trump and Ukraine President Zelenskiy. “Assuming President Z convinces trump he will investigate … we will nail down date for visit to Washington.”
Ukraine’s Yermak responds after the call, noting that it went well. But the issue remained unresolved.
“I think it’s possible to make this declaration and mention all these things… But it will be logic to do after we receive a confirmation of date,” Yermak wrote Aug. 10.
Volker later says he agrees with that approach.
Volker never mentions Biden, but he does 'Burisma'
Volker makes clear in his texts to Yermak on exactly what the U.S. wants to hear, going so far as to help draft a statement for Ukraine. In that statement, Volker writes, Ukraine’s president would announce plans to examine potential 2016 election meddling – an apparent reference to claims by some conservatives that corrupt Ukranian officials tried to help Hillary Clinton win.
Volker’s texts also call on Ukraine to say it would look into the dealings of Burisma, an energy company that employed Biden’s son, Hunter, on its board.
At one point, Volker sends a proposed draft statement to his colleague, Sondland, who responds: "Perfect. Lets send to Andrey after our call."
In his prepared testimony to Congress, Volker says Biden is never mentioned specifically. He also says suggestions that Biden acted improperly “has no credibility to me.”
A career civil service officer challenged his colleagues, calling Trump's actions 'crazy'
Of the three, it's Taylor -- the top diplomat in Ukraine who has served in at least three administrations, focusing on international affairs -- who expresses serious concern that the administration has placed a hold on aid to Ukraine, warning the move had "shaken their faith with us."
At the time of the exchange, questions were already swirling about Trump's aid strategy. News broke in late August that nearly $400 million of security assistance had been put on hold by the White House.
In a Sept. 5 Washington Post editorial, the newspaper said it had received information Trump was "attempting to force" Ukraine's president "to intervene in the 2020 U.S. presidential election by launching an investigation of the leading Democratic candidate, Joe Biden."
After describing the prospect of Ukraine not getting the aid as a "nightmare," Taylor says: "I think it's crazy to withhold security assistance for help with a political campaign."
The diplomat who denied any 'quid pro quo' was a Trump megadonor
Sondland, the top U.S. representative to the European Union, swiftly knocks down Taylor's allegation with a formal-sounding statement.
"Bill, I believe you are incorrect about President Trump's intentions," he writes via text. "The President has been crystal clear: no quid pro quo's of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promised during his campaign."
Sondland's statement is likely to be seen by Republicans as proof the administration wasn't looking to tie foreign aid in return for a promise by Ukraine to investigate a Trump political rival.
Democrats, however, are going to note Sondland's political background as a Republican megadonor and hotelier who contributed more than $1 million to Trump's inaugural committee before being appointed to his first diplomatic post. He was confirmed by the Senate in 2018. They will also likely focus on his suggestion to Taylor to take the conversation offline.
This is just the beginning
The encrypted texts provided to Congress and leaked Thursday to reporters is likely just the beginning of this line of the investigation. They came from Volker, who testified behind closed doors Thursday after resigning last week. While there are exchanges between the men over a number of days, it's unclear whether the texts show their complete communication.
The House Intelligence Committee on Friday was expected to hear from the intelligence community's inspector general, and other State Department officials are scheduled for depositions in the coming weeks.
ABC News' Katherine Faulders and Conor Finnegan contributed to this report.