Washington (CNN) The impeachment inquiry has uncovered at least three examples of the quid pro quo between the Trump administration and Ukraine, where US military aid and a White House visit were used as leverage to secure an announcement that Ukraine was investigating President Donald Trump's rivals, according to documents and testimony from key witnesses.

The question of whether there was a quid pro quo is at the heart of the impeachment inquiry

Trump has been adamant that he did nothing wrong and tweeted at least 15 times since the inquiry began that there was no quid pro quo. Yet many Democrats have said from the start that they saw evidence of Trump attempting to trade US military assistance for political favors from Ukraine.

Legal analysts and experts on the impeachment process have said the investigation doesn't actually need to find incontrovertible proof of a quid pro quo for the House to impeach Trump.

Nevertheless, after a month of interviews with senior Trump administration officials, lawmakers have unearthed at least three examples of the quid pro quo.

Here's a complete breakdown:

The Volker text messages

Trump's special envoy for Ukraine, Kurt Volker, told a senior Ukrainian official that the new Ukrainian President could secure a White House invite if he convinced Trump he would launch an investigation into potential Ukrainian meddling in the 2016 US presidential election.

This happened in a text message from Volker to Andrey Yermak , a senior aide to Ukrainian President Volodymyr Zelensky, a political newcomer who never held elected office before this year. It was sent shortly before an important call on July 25 between Trump and Zelensky.

Volker told Yermak in the text: "Good lunch -- thanks. Heard from White House — assuming President Z convinces trump he will investigate / 'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington. Good luck! See you tomorrow -kurt."

Trump has never never fully accepted the fact that the Russian government interfered in the 2016 presidential election, hacking Democrats and leaking their emails to help him win. Instead, he has promoted unfounded conspiracy theories that Ukraine framed the Russians.

By asking Ukraine to explore these theories , the Trump administration was seeking information that could undermine the Russia investigation, which would be politically beneficial to Trump.

The Trump phone call

After Volker sent those texts, Trump got on the phone with Zelensky . During the July 25 conversation, Zelensky brought up US military assistance to Ukraine, which has been at the center of US policy since Russia and its proxies invaded Eastern Ukraine in 2014. Zelensky told Trump he was interested in buying additional anti-tank missiles that Ukraine could have in its arsenal.

"I would like you to do us a favor though," Trump interjected, according to a rough White House transcript, "because our country has been through a lot and Ukraine knows a lot about it. I would like you to find out what happened with this whole situation with Ukraine, they say CrowdStrike ... I guess you have one of your wealthy people ... The server, they say Ukraine has it."

This was the request for Ukraine to investigate the conspiracy theories about 2016. CrowdStrike is the California-based cybersecurity firm that helped the Democratic National Committee figure out that Russia was responsible for the hacking. During the call, Trump mentioned the unfounded theory that the DNC's hacked servers were somehow hidden in Ukraine.

Later in their conversation, Trump asked Zelensky to work with his personal attorney Rudy Giuliani to investigate allegations of corruption by former Vice President Joe Biden and his son Hunter, who had a highly-paid position on the board of a Ukrainian energy company. Despite Trump and Giuliani's claims there is no evidence of wrongdoing by the Bidens.

Trump never committed to the anti-tank missiles Zelensky requested during the call. Instead, Zelensky said he would look into the two politically charged matters that Trump brought up.

The Sondland pull-aside

As the summer progressed, a Trump-Zelensky meeting at the White House still had not been scheduled, and news reports revealed that there was an inexplicable holdup in US military and security aid for Ukraine -- a $400 million package that was already approved by Congress.

Trump was meant to meet Zelensky at an event in Poland, but those plans fell through when Trump stayed home to deal with a hurricane. Zelensky met with Vice President Mike Pence instead. After that meeting, Zelensky's aide Yermak had a conversation with US Ambassador to the European Union Gordon Sondland, a wealthy donor to Trump who handled Ukraine issues.

After first denying there was ever a quid pro quo offered to Ukraine, Sondland made a significant revision to his testimony this week in which he admitted that there was.

"I now recall speaking individually with Mr. Yermak, where I said that resumption of U.S. aid would likely not occur until Ukraine provided the public anti-corruption statement that we had been discussing for many weeks," Sondland said in his revised testimony to House lawmakers.

The "anti-corruption statement" is an apparent euphemism for the public announcement that Trump hoped Zelensky would make, where Zelensky would say that Ukraine was investigating the allegations regarding Biden and his son, as well as potential Ukrainian meddling in 2016. A declaration like that would give Trump a major talking point to use against Biden on the trail.