Controversy has brewed for months involving former Vice President Joe Biden, his son Hunter, Ukrainian authorities, Rudy Giuliani, and President Donald Trump.

On Tuesday, House Speaker Nancy Pelosi announced an impeachment inquiry over allegations that Trump pressured Ukrainian officials to investigate his potential 2020 rival for his own political gain.

At the heart of the conflict are Trump's claims that Joe Biden pushed for Ukraine's president to fire the prosecutor investigating a gas company whose board Hunter Biden served on.

But multiple reports show Trump and Giuliani wanted to pressure Ukrainian authorities into pursuing investigations that could politically benefit Trump.

On Wednesday, the White House released a memo of Trump's call with Ukrainian President Volodymyr Zelensky, showing that Trump pressured Zelensky to work with US officials to investigate the Bidens.

Ukrainian officials have said previously there was not an active investigation into the younger Biden and there was no evidence of wrongdoing against either Biden.

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A mysterious whistleblower complaint filed against President Donald Trump has revived a controversy over former Vice President Joe Biden's son Hunter's ties to a business that was under investigation by Ukrainian authorities.

In recent days, the Trump administration has sparked an uproar over Trump's handling of a call with Ukrainian President Volodymyr Zelensky, in which he urged the foreign leader to investigate Biden and his son. The call came just days after Trump withheld nearly $400 million in aid to Ukraine.

On Tuesday, House Democrats announced an impeachment inquiry over allegations that Trump pressured foreign officials to investigate his potential 2020 Democratic opponent for his own political gain.

Read more: Trump is facing impeachment over a whistleblower complaint and a phone call with Ukraine's president. Here's what we know.

U.S. President Donald Trump speaks on the phone with Prime Minister of Australia Malcolm Turnbull in the Oval Office of the White House, Saturday, Jan. 28, 2017 in Washington. Associated Press/Alex Brandon

Then on Wednesday, the White House released a memo of the July 25 call, revealing that Trump urged Zelensky to launch an investigation and instructing him to speak with US Attorney General William Barr and Rudy Giuliani, Trump's lawyer.

"There's a lot of talk about Biden's son. That Biden stopped the prosecution and a lot of people want to find out about that, so whatever you can do with the Attorney General would be great," Trump said, according to the memo. "Biden went around bragging that he stopped the prosecution so if you ·can look into it ... It sounds horrible to me."

Giuliani has been a central figure in the controversy. He has reportedly tried to meddle with foreign policy between the US and Ukraine and has appeared frequently on cable-news shows in the last week denouncing the Bidens and talking up an investigation.

Read more: Chris Cuomo and Rudy Giuliani had a 28-minute shouting match over the whistleblower complaint, and Giuliani acknowledged telling Ukraine to investigate Biden

A May report raised questions about a possible conflict of interest and if Biden was trying to shield his son

Hunter Biden Paul Morigi/Getty Images for World Food Program USA

Trump and Giuliani have pushed claims for months that the former vice president bribed government officials in Ukraine to stifle investigations into Burisma, the gas company for which Hunter was a board member.

The claims go back to a bombshell New York Times story published May 1 that detailed both Bidens' ties to Ukraine, and revealed that the former vice president had successfully gotten a Ukrainian prosecutor removed from office.

The development raised questions over a possible conflict of interest and suspicions that Biden was trying to shield his son from an investigation into one of his business interests.

Hunter Biden worked for the Boies Schiller Flexner law firm that consulted for Burisma. Biden was in charge for the company's legal affairs with international companies, according to a press release, but Biden has disputed that characterization. At the time, Burisma's founder faced multiple investigations into alleged tax evasion and money laundering.

Hunter Biden later stepped down from the board in April 2019 when his term expired.

Joe Biden was part of a larger push coming from former President Barack Obama's administration for the Ukrainian prosecutor Viktor Shokin to be removed over concerns of corruption in his office. Biden acted as a vocal opponent of Shokin and has publicly detailed his threat to withhold $1 billion in loan guarantees from the country if Shokin wasn't fired.

But reports have since emerged showing there was not an active investigation into Hunter when the former vice president made the push for Shokin's firing. Yuriy Lutsenko, Ukraine's prosecutor general, later said that he had no evidence of wrongdoing against either Biden.

Trump and Giuliani have been pushing a narrative suggesting that the Biden's were involved in corruption

Rudy Giuliani endorses Marco Rubio for Florida's open Senate seat during a campaign rally in Miami. J Pat Carter/AP

Despite the absence of proven wrongdoing by the Bidens, the Trump administration has used the back-and-forth to obscure Giuliani's communications with Ukraine and growing concerns that Trump may have collaborated with a foreign power to gain political leverage.

After the report, Giuliani announced he was planning to travel to Ukraine to press president-elect Volodymyr Zelensky to pursue two investigations that could act as political leverage for Trump.

Giuliani eventually canceled his trip after facing widespread criticism for what appeared to be an attempt to push a foreign government to meddle in the 2020 US election, but Trump and his allies have continued claiming Biden was acting in his own personal interest.

Trump later tweeted a video with news coverage of the initial report questioning Bidens' intentions as a part of his campaign marketing efforts.

Trump's contact with Ukraine could be at the center of the whistleblower complaint

Joe Biden may run against Trump in the 2020 general election. Associated Press

The Washington Post reported that two former US officials said Ukraine was at the center of the complaint, and a July 25 phone call between Trump and president-elect Volodymyr Zelensky has come into focus as a key potential piece in tracing the administration's efforts to influence Ukranian officials.

Read more: A mysterious exchange between Trump and a foreign leader is Washington's latest obsession. Here's what is actually going on.

According to an official statement on Zelensky's website from the time the July 25 call centered around Trump offering his congratulations to Zelensky over his recent election to the presidency. However, the Wall Street Journal reported that Trump pressed Zelensky to probe Hunter Biden, at least eight times, according to people familiar with the conversation.

Trump dismissed the reports, saying that he has "conversations with many leaders."

"It's always appropriate, always appropriate," Trump said of his conversations. "At the highest level always appropriate. And anything I do, I fight for this country."

In a series of tweets in the days after the complaint was revealed, Trump denied the reports and repeated his claims about Biden executing a bribe.