Here is a timeline of events in the Ukraine scandal, which has prompted the opening of an impeachment inquiry by the Democratic-led US House of Representatives into US President Donald Trump:
Trump holds a 30-minute telephone conversation with Ukraine's new president Volodymyr Zelensky, a former professional comedian who was elected in May.
Earlier in July, Trump, without explanation, suspended hundreds of millions of dollars in military assistance to Ukraine, which had been approved by Congress.
The aid is eventually released by the White House on 11 September.
An anonymous whistleblower in the US intelligence community files an internal complaint about the Trump-Zelensky call, describing it as a matter of "urgent concern".
The Washington Post publishes a report about the whistleblower complaint, saying that it involves a "promise" made by Trump to a foreign leader.
Michael Atkinson, the Inspector General of the Intelligence Community, testifies before the House Intelligence Committee about the whistleblower complaint, but declines to reveal its contents, saying that acting director of national intelligence Joseph Maguire has barred him from doing so.
Trump reacts for the first time, calling the Post story "fake news" and stating that when he speaks with foreign leaders, he is well aware that there are likely "many people" listening.
Trump confirms that he discussed Joe Biden, his potential Democratic rival in the 2020 presidential election, and Biden's son, Hunter, in his call with Zelensky.
Hunter Biden served on the board of a Ukrainian gas company while his father was vice president, but has not been officially accused of any wrongdoing.
Amid a cascade of press reports that Trump pushed Zelensky to investigate the Bidens, Nancy Pelosi, Democratic speaker of the House of Representatives, announces the opening of an impeachment inquiry of the president for abuse of power.
"The president must be held accountable," Pelosi says. "No one is above the law."
Trump calls it "PRESIDENTIAL HARASSMENT."
The White House releases a rough transcript of the Trump-Zelensky call.
It confirms that the president repeatedly asked the Ukrainian leader to investigate the Bidens and to "look into" it with Rudy Giuliani, Trump's personal lawyer, and with Attorney General Bill Barr.
The House Intelligence Committee releases the whistleblower's complaint, which accuses Trump of "using the power of his office to solicit interference from a foreign country in the 2020 US election".
It also accuses White House lawyers of seeking to "lock down" access to the call transcript because of its politically sensitive nature.
Pelosi and several 2020 Democratic presidential candidates accuse the White House of engaging in a "cover-up".
House committees subpoena Secretary of State Mike Pompeo for Ukraine-related documents.
Kurt Volker, the US special representative on Ukraine, resigns after Congress orders him to provide a deposition as part of the impeachment inquiry.
House Democratic leaders order Giuliani to turn over Ukraine-related documents by 15 October.
Pompeo pushes back against the House subpoenas, accusing Democrats of trying to "bully" US diplomats into testifying against the White House.
Trump steps up his personal attacks on Adam Schiff, the chairman of the House Intelligence Committee who is spearheading the impeachment probe, saying he should be arrested for "treason".
Trump openly calls on China as well as Ukraine to investigate the Bidens and dismisses his Democratic opponents as "maniacs" pursuing "impeachment crap".
Volker testifies behind closed doors on Capitol Hill.
Text messages between US diplomats are released indicating the Trump administration tried to tie military aid to Ukraine to an investigation into the Bidens.