A look at the events of the controversy that has led House Democrats to launch a formal impeachment inquiry of Trump.

US President Donald Trump’s interactions with Ukraine have prompted an extraordinary whistle-blower complaint that the Republican used his powers to pressure a foreign leader to investigate one of his chief Democratic rivals.

An unclassified version of the complaint was released on Thursday. Reports of the complaint and Trump’s comments surrounding it prompted Democratic House Speaker Nancy Pelosi to launch a formal impeachment inquiry into Trump.

She has since said the complaint shows Trump has “betrayed his oath of office, our national security and the integrity” of US elections.

Trump has denied any wrongdoing and called the impeachment inquiry a “hoax” and “witch-hunt garbage”.

Here is a timeline of events in the controversy based on the whistle-blower’s complaint and public reports.

Late 2018 – Trump’s personal lawyer Rudy Giuliani speaks to former Ukrainian Prosecutor General Viktor Shokin by Skype. Shokin, widely accused of corruption, had been removed from his job in 2016.

Late January 2019 – Giuliani meets with then-Ukrainian Prosecutor General Yuriy Lutsenko in New York.

Mid-February– Giuliani meets Lutsenko in Warsaw.

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Late March – Lutsenko and other Ukrainian officials begin publishing series of articles on Washington website The Hill alleging that their political rivals had worked with the Democratic National Committee and US Embassy in Kiev to interfere in the 2016 presidential election, and that former Vice President Joe Biden had pressured the former Ukrainian president to fire a prosecutor looking into a gas company connected with his son, Hunter Biden.

April 21 – Volodymyr Zelenskyy wins Ukrainian election, beating incumbent Petro Poroshenko. Trump calls Zelenskyy to congratulate him and discusses “reforms that strengthen democracy, increase prosperity, and root out corruption” according to a White House readout of the call.

May 6 – State Department announces ambassador to Ukraine Masha Yovanovitch – a career diplomat – would end her assignment in Kiev. Democratic politicians immediately call the decision politically motivated.

May 9 – The New York Times reports Giuliani plans to travel to Ukraine to press the government to investigate interference in 2016 election and the Biden family.

May 10 – Trump tells Politico he plans to speak with Giuliani about the trip. Giuliani later cancels the trip, claiming Zelenskyy was “surrounded by enemies” of Trump.

May 14– In a published interview with a Ukrainian journalist, Giuliani says Yovanovitch had been removed because she had been working against Trump.

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Around May 14 – Trump instructs Vice President Mike Pence to cancel plans to attend Zelenskyy’s inauguration. Instead, Energy Secretary Rick Perry led the small US delegation.

Mid-May – Whistle-blower hears from multiple officials that Kurt Volker, the US special representative to Ukraine, and Gordon Sondland, US ambassador to the European Union, had spoken with Giuliani to try to “contain the damage” from his efforts. Volker and Sondland also met with Zelenskyy’s administration to talk about the issues.

May 20 – Zelenskyy is inaugurated as president.

June 21 – Giuliani complains in a tweet that Zelenskyy “still silent on investigation of Ukrainian interference in 2016 and alleged Biden bribery” of Poroshenko.

July 18 – White House Office of Management and Budget informs departments and agencies that Trump had issued instructions earlier that month to suspend all US security assistance to Ukraine.

July 25 – Trump has a phone call with Zelenskyy and asks him to speak to Attorney General William Barr and Giuliani about reopening a Ukrainian investigation into a Ukrainian gas company connected to Hunter Biden.

July 26– Volker and Sondland meet with Zelenskyy and other Ukrainian political figures and “reportedly provided advice to the Ukrainian leadership about how to ‘navigate’ the demands that the president had made”, according to the whistle-blower report released on Thursday.

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August 2 – On or around this date, Giuliani travels to Madrid to meet Zelenskyy adviser Andriy Yernak, which the whistle-blower said was “a ‘direct follow-up'” to the July 25 Trump-Zelenskyy phone call.

August 8 – Giuliani tells Fox News that US Attorney John Durham was investigating Ukraine. Durham had been named by Attorney General William Barr as part of a probe into the origins of the investigation into Russian election meddling and connects to the Trump campaign.

August 12 – The whistle-blower files complaint.

September 18 – The Washington Post publishes first public account about the whistle-blower complaint.

September 25 – Justice Department releases summary of Trump-Zelenskyy call. Trump and Zelenskyy meet in person for the first time at a UN gathering in New York.

September 26 – House Intelligence Committee releases unclassified version of whistle-blower complaint.