(CNN) The White House on Friday released its transcript of President Donald Trump's April 21 phone call with Ukrainian President Volodymyr Zelensky congratulating him on his election.

Here's a timeline of key events between that first call between the two and their July 25 call, during which Trump pressured Zelensky to open investigations that could damage former Vice President Joe Biden heading into the 2020 election. For more, here's a detailed timeline

April 25: Former Vice President Joe Biden announces he is running for President. His son Hunter Biden leaves the board of Burisma, the Ukrainian natural gas company where he's held a position since 2014.

About April 29: The whistleblower learns that US Ambassador to Ukraine Marie Yovanovitch, a career Foreign Service officer, had been recalled to Washington and that associates of Trump's personal attorney Rudy Giuliani were trying to contact Zelensky's incoming team.

May 1: publishes a report on Hunter Biden and Burisma and Giuliani's push for Ukrainians to investigate the two. Giuliani met repeatedly with Ukrainian Prosecutor General Yuriy Lutsenko in New York, according to the report, which also notes Giuliani has talked about his theories with Trump. The New York Times publishes a report on Hunter Biden and Burisma and Giuliani's push for Ukrainians to investigate the two. Giuliani met repeatedly with Ukrainian Prosecutor General Yuriy Lutsenko in New York, according to the report, which also notes Giuliani has talked about his theories with Trump.

May 6: The US State Department announces that Yovanovitch will permanently leave her post in the country on May 20.

May 10: Giuliani draws scrutiny for saying he will travel to Ukraine to meet with Zelensky and then cancels the trip.

May 16: Lutsenko, the Ukrainian prosecutor general, tells Lutsenko, the Ukrainian prosecutor general, tells Bloomberg there's no evidence of any wrongdoing in Ukraine by the Bidens.

June 12: Trump Trump tells ABC's George Stephanopoulos that he would want to hear it if foreign governments offered him damaging information about his opponents in the 2020 presidential election.

June 21: Giuliani Giuliani tweets that Zelensky was "still silent on investigation of Ukrainian interference" in the 2016 US presidential election.

July 10: During a contentious meeting at the White House, it becomes clear to National Security Council Russia expert Fiona Hill and NSC Ukraine expert Alexander Vindman that an Oval Office visit for Ukraine's president was contingent on him opening an investigation into Trump's political rivals.

Hill told lawmakers that Gordon Sondland, the US Ambassador to the European Union, said there was an agreement with acting White House chief of staff Mick Mulvaney that "they would have a White House meeting or, you know, a Presidential meeting, if the Ukrainians started up these investigations again."

Separately, Vindman testified that Sondland characterized the request for investigations by Ukraine -- or a "deliverable" -- as coordinated with Mulvaney, according to the transcript of his testimony.

July 24: Special counsel Robert Mueller Special counsel Robert Mueller testifies to Congress about the probe into Russian interference in the 2016 presidential election, which Trump has called a witch hunt.

Mueller details multiple interactions between Trump campaign officials and Russians during the campaign, but says he has not found evidence of a criminal conspiracy involving the Trump campaign and that Trump himself, according to Department of Justice policy, cannot be prosecuted.

Mueller also details possible instances of obstruction of justice when Trump tried to squash the probe. Trump hails the testimony as vindication, but the fact that Mueller couldn't prosecute him along with the documentation of possible obstruction makes calls for Trump's impeachment grow.

July 25: Trump and Zelensky talk by phone. A readout of the call between Trump and Zelensky is posted on the official website of the President of Ukraine and it includes that "Donald Trump is convinced that the new Ukrainian government will be able to quickly improve image of Ukraine, complete investigation of corruption cases, which inhibited the interaction between Ukraine and the USA."