Timeline: What led to impeachment inquiry

President Donald Trump repeatedly pressured the president of Ukraine to investigate people and issues of political concern to Trump, including former Vice President Joe Biden and his son Hunter. Here's a timeline of events since January.

April 21: Trump calls Ukraine's president-elect, Volodymyr Zelensky, to congratulate him and asks Zelensky to pursue investigations of "corruption."

April 25: On Fox News, Trump suggests he would like Attorney General William Barr to review information gathered by Ukrainian prosecutors about what network host Sean Hannity suggests was Ukrainian "collusion" with Democrats in 2016.

May 6: The Trump administration recalls the United States ambassador to Ukraine, Marie Yovanovitch. Conservative allies of Trump had come to believe she was not sufficiently supportive of the president.

May 10: Trump tells Politico that it would be "appropriate" for him to speak to Barr about launching an investigation into the Bidens' work in Ukraine, and that he plans to speak with his lawyer Rudy Giuliani about his planned trip to Ukraine.

Mid-May: Trump tells Vice President Mike Pence to cancel a trip to attend Zelensky's inauguration, a whistleblower alleged in a complaint released Thursday. Other administration officials, including Rick Perry, the secretary of energy, attend instead.

May 23: After a U.S. delegation returns from a trip to Zelensky's inauguration, Trump is briefed on their favorable impressions of the new Ukrainian leader. Trump responds that he thinks all Ukrainian politicians are corrupt.

Mid-July: Trump instructs Mick Mulvaney, the acting White House chief of staff, to hold back almost $400 million in aid to Ukraine.

July 25: Trump speaks to Zelensky by phone and asks for a "favor": Ukraine's help in looking into an unfounded conspiracy theory holding that Ukrainians — and not the Russian government — were involved in the hacking of the Democratic National Committee. Trump goes on to cite his desire to look at Biden and his son and suggests that Zelensky work with Giuliani and Barr.

Sept. 11: The White House informs senators that it is releasing the Ukraine military assistance it had been withholding.

Sept. 22: Trump acknowledges that he raised corruption accusations against Biden during a phone call with Ukraine's leader.

January: Giuliani meets in New York for hours with a Ukrainian prosecutor. Giuliani pushes for information about — and investigations into — Trump's rivals, including Biden. Giuliani briefs Trump on his efforts.

May 9: The New York Times reports that Giuliani was planning a trip to Kiev to push the incoming government to press ahead with investigations into Biden and into possible Ukrainian connections to the origins of special counsel Robert Mueller's probe.

A few hours later: Giuliani cancels his trip to Kiev.

July 10: Over breakfast at the Trump Hotel, Zelensky's aide, Andriy Yermak, asks Kurt Volker, the U.S. special envoy for Ukraine, to connect him to Giuliani.

A few days later: Yermak talks to Giuliani by phone for the first time.

After Trump's call with Zelensky: According to the complaint, White House officials are directed to move the transcript of the call into an electronic system that is normally used for sensitive, classified information — "not the first time" a transcript is placed there to protect politically sensitive material.

Early August: Giuliani meets in Madrid with a top aide to Zelensky to encourage the Ukrainian government to investigate Trump's political opponents.

Sept. 9: The intelligence community inspector general sends a letter to the House and Senate Intelligence Committees informing them of the existence of a whistleblower complaint.

Sept. 9: The House Foreign Affairs, Intelligence and Oversight Committees launch an investigation into whether Trump and Giuliani are abusing the U.S. foreign policy apparatus for political gain.