(Reuters) - Democrats in the U.S. House of Representatives began an inquiry on Sept. 24 to determine whether President Donald Trump abused his office for political gain when he asked Ukrainian President Volodymyr Zelenskiy in a July phone call to investigate political rival Joe Biden, a former vice president.

FILE PHOTO: U.S. President Donald Trump speaks to the media on the South Lawn of the White House in Washington, U.S., before his departure to New York, November 2, 2019. REUTERS/Yuri Gripas

The contents of the call were revealed in a whistleblowercomplaint by an intelligence official. Testimony by Trump administration officials past and present, a rough transcript of the phone call released by the White House, texts between U.S. diplomats and other documents have largely confirmed the whistleblower's account. (Graphic on inquiry: here)

Trump denies wrongdoing and says he is the victim of a witch hunt by Democrats.

Here’s what we know so far:

- A rough transcript of the call on July 25 between Trumpand Zelenskiy confirmed the whistleblower’s most damagingallegation - that Trump asked Zelenskiy to investigate Burisma,a Ukrainian energy company on which Hunter Biden, the son of JoeBiden, had served as a board member. Trump, aRepublican, also asked Zelenskiy to “do us a favor” andinvestigate a debunked conspiracy theory that a hackedDemocratic National Committee computer server was in Ukraine,according to the transcript.

- Text messages between Trump’s Ukraine special envoy, KurtVolker, his European Union Ambassador Gordon Sondland and hispersonal attorney, Rudy Giuliani, show that pressure was exertedon Zelenskiy to make a public statement committing himself toinvestigating Burisma before he would be allowed to meet withTrump at the White House, part of the “quid pro quo” - Latinfor a favor for a favor - that is at the heart of theimpeachment inquiry.

- Sondland, a hotelier and Trump donor, testified tocongressional investigators that Trump largely delegated Ukrainepolicy to Giuliani. He said Trump told him and other officialsat a White House meeting to coordinate with Giuliani, who at thetime was seeking to dig up dirt on Biden, a leading candidatefor the Democratic presidential nomination in 2020. Sondland expressed disquiet in his testimony about allowing a private citizen to have such an influential role in U.S. foreign policy.

- In testimony considered the most damning to date, the topU.S. diplomat in Ukraine, William Taylor, said Trump made therelease of U.S. security aid to Ukraine contingent on Kievpublicly declaring it would carry out the investigations thatthe U.S. president sought.

Trump has contended that he did not hold up the $391 million in U.S. military aid to pressure Zelenskiy. Taylor also said Trump had made a White House visit by Zelenskiy contingent on his opening the investigations.

- In remarks on Oct. 17 that stunned many in Washington, Trump’s acting chief of staff, Mick Mulvaney, acknowledged thatthe aid to Ukraine was indeed linked to Trump’s request forinvestigations into the debunked conspiracy theory and HunterBiden. Mulvaney later contradicted himself in astatement from the White House that ruled out a quid pro quo.

- The former U.S. ambassador to Ukraine, Marie Yovanovitch,testified that Trump had ousted her from her position based on“unfounded and false claims” after she had come under attack byGiuliani. She was abruptly recalled from Kiev in May and told that Secretary of State Mike Pompeo could not protect her from Trump any longer, according to a transcript of her testimony. She said she felt threatened by Trump describing her on his call with Zelenskiy as “bad news.”

- Volker, the former special envoy to Ukraine, testifiedthat he had helped to connect Giuliani with a top aide toUkraine’s president as the president’s personal lawyer continuedto seek information damaging to the Bidens. Volker said he wasunaware of Giuliani’s mission at the time and that in the nowreleased text messages between him, Sondland and Giuliani therewas no explicit mention of the Bidens.

- Michael McKinley, a former adviser to Secretary of StateMike Pompeo, testified that he quit a few days before hisappearance to congressional committees because of departmental leadership’s unwillingness to defend Yovanovitchfrom the attacks on her. He also objected to what he said was the Trump administration using ambassadors to advance domestic political objectives, according to a transcript of his testimony.

- Trump’s former national security adviser John Boltonexpressed alarm about Giuliani’s involvement in Ukraine policyand the efforts to press Zelenskiy to give Trump political help,the U.S. president’s former Russia adviser Fiona Hill testified.Democratic investigators want to talk to Bolton.

- A top adviser to Trump on Ukraine has testified that he was so alarmed after hearing Trump ask Ukraine’s president to investigate Biden in the July 25 phone call that he reported the matter to a White House lawyer out of concern for U.S. national security. Army Lieutenant-Colonel Alexander Vindman said the lawyer, John Eisenberg, took the unusual step of moving a transcript of the call into the White House’s most classified computer system.

- Two foreign-born Florida businessmen who helped Giulianiinvestigate the Bidens in Ukraine have been indicted for ascheme to illegally funnel money to a pro-Trump electioncommittee and other U.S. political candidates. They have pleadednot guilty.