(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.

U.S. President Donald Trump speaks during a "Black Voices for Trump" campaign event in Atlanta, Georgia, U.S., November 8, 2019. REUTERS/Jonathan Ernst

The contents of the call were revealed in a whistleblower complaint 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 Trump and Zelenskiy confirmed the whistleblower’s most damaging allegation: that Trump asked Zelenskiy to investigate whether Biden, while vice president, blocked an anti-corruption probe into Burisma, a Ukrainian energy company on which his son Hunter Biden had served as a board member. No evidence has emerged to support that claim. Trump, a Republican, also asked Zelenskiy to “do us a favor” and investigate a debunked conspiracy theory that a hacked Democratic National Committee computer server was in Ukraine, according to the transcript.

- Text messages between Trump’s Ukraine special envoy, Kurt Volker, his European Union Ambassador Gordon Sondland and his personal attorney, Rudy Giuliani, show that pressure was exerted on Zelenskiy to make a public statement committing himself to investigating Burisma before he would be allowed to meet with Trump at the White House, part of the “quid pro quo” - Latin for “a favor for a favor” - that is at the heart of the impeachment inquiry.

- Sondland, a hotelier and Trump donor, testified to congressional investigators that Trump largely delegated Ukraine policy to Giuliani. He said Trump told him and other officials at a White House meeting to coordinate with Giuliani, who at the time was seeking to dig up dirt on Biden, a leading candidate for the 2020 Democratic presidential nomination. 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 top U.S. diplomat in Ukraine, William Taylor, said Trump made the release of U.S. security aid to Ukraine contingent on Kiev publicly declaring it would carry out the investigations that the 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 that the aid to Ukraine was indeed linked to Trump’s request for investigations into the debunked conspiracy theory and Hunter Biden. Mulvaney later contradicted himself in a statement 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 by Giuliani. 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, testified that he had helped to connect Giuliani with a top aide to Ukraine’s president as the president’s personal lawyer continued to seek information damaging to the Bidens. Volker said he was unaware of Giuliani’s mission at the time and that in the now-released text messages between him, Sondland and Giuliani there was no explicit mention of the Bidens.

- Michael McKinley, a former adviser to Pompeo, testified that he quit a few days before his appearance before congressional committees because of departmental leadership’s unwillingness to defend Yovanovitch from 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 Bolton expressed alarm about Giuliani’s involvement in Ukraine policy and the efforts to press Zelenskiy to give Trump political help, the U.S. president’s former Russia adviser Fiona Hill testified. Bolton has said he will not testify until a federal court decides whether current and former administration officials like him must cooperate with the investigation.

- 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.

- Vindman said he also heard Sondland pressure Ukrainian officials to investigate the Bidens and Burisma in order to get a coveted meeting with Trump. Sondland said he pushed for an investigation into Burisma, but did not know that it was related to the Bidens.

- Two foreign-born Florida businessmen who helped Giuliani investigate the Bidens in Ukraine have been indicted for a scheme to illegally funnel money to a pro-Trump election committee and other U.S. political candidates. They have pleaded not guilty.