(CNN) The impeachment inquiry centers on claims that President Donald Trump used US tax dollars as bait to pressure Ukrainian President Volodymyr Zelensky to open investigations that could damage former Vice President Joe Biden heading into the 2020 election.

Below are the key dates in the Democratic case. For more, here's a detailed timeline

April 21 -- Zelensky is elected in Ukraine

Zelensky, a comedian and actor who previously played the President of Ukraine on TV, wins a landslide victory to become the actual President of Ukraine . Ukraine, which is in the midst of a war with Russian-led forces, badly needs US and other Western aid, and Zelensky quickly seeks a meeting with US President Donald Trump.

May 1 -- Rudy Giuliani's theory goes mainstream

The New York Times publishes a report on Hunter Biden and the Ukrainian company Burisma, which had placed him on its board while his father was still US vice president. Trump's personal lawyer Rudy Giuliani has been pushing for Ukrainians to investigate the arrangement and 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.

July 10 -- White House meeting between US and Ukrainian officials

This meeting has emerged as a key moment because it represents the first time it was clear that members of the administration were requiring a quid pro quo for Trump meeting with Zelensky, according to people giving testimony to the impeachment inquiry. The meeting featured White House and State Department officials as well as top officials in the Ukrainian government.

Fiona Hill, a National Security Council Russia expert, testified that the suggestion alarmed then-national security adviser John Bolton, who "immediately stiffened" and ended the meeting.

Separately, top National Security Council expert on Ukraine Alexander Vindman testified that Ambassador to the European Union Gordon Sondland characterized the request for investigations by Ukraine as a "deliverable" -- and coordinated with Mulvaney, according to the transcript of his testimony.

Before July 25 -- Aid to Ukraine frozen

July 25 -- Phone call between Trump and Zelensky

Trump and Zelensky talk by phone, officially so that Trump can congratulate Zelensky on successful parliamentary elections. Trump repeatedly brings up investigations he'd like Zelensky to look into, including of interference in the 2016 election and the relationship between Burisma and the Bidens. Read an annotated transcript of the call

A readout of the call between Trump and Zelensky posted on the official website of the President of Ukraine includes a note 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."

The call triggers immediate concerns in the White House, including on the part of Vindman, who listened in. The transcript of the call is placed in a more secure server.

July 26 -- Diplomats meet with Zelensky; Trump asks Sondland about investigations

US Special Envoy for Ukraine Kurt Volker, accompanied by Sondland, meets with Zelensky in Kiev. They discuss military topics, according to Zelensky's statement after the meeting, which does not mention the corruption investigations.

Volker and Sondland reportedly advise the Ukrainian leadership on how to "navigate" Trump's demands of Zelensky, according to the later whistleblower complaint.

An aide to Bill Taylor, the top US diplomat in Ukraine, overhears a phone call between Trump and Sondland in which Trump asks about the investigations, according to Taylor's later testimony.

Separately, Pentagon aides, according to the Cooper testimony, realize the holdup in aid is directly related to Trump's concerns about Ukraine and begin trying to legally justify holding the aid , which had been allocated by Congress.

August 1 -- Giuliani meets Ukrainian official

Following up on Trump's request to Zelensky, Giuliani and Zelensky's aide Andriy Yermak meet in Madrid, according to Giuliani.

August 12 -- Whistleblower files complaint

This document will eventually trigger the impeachment inquiry. Read an annotated version

August 29 -- Ukraine raises concerns about the aid

According to Taylor's testimony, he speaks on the phone with Yermak, who is alarmed about the status of the aid. Yermak separately texts Volker a Politico story on the withheld aid.

September 1 -- Sondland delivers ultimatum; Taylor raises concerns about Ukraine aid

Sondland recalls telling Yermak, on the sidelines of a meeting between Vice President Mike Pence and Zelensky in Warsaw, that there would be no aid without the investigations

Separately, Taylor texts Sondland and asks if security assistance, and not just a White House meeting, is conditioned on the investigations. In a subsequent phone call, Sondland, says Taylor, makes clear that everything is going to be withheld because Trump wants to put the Ukrainians in a "public box" and force the investigations on them.

September 9 -- Concerns from Taylor and the inspector general come to a head

In text messages and in a phone call with Sondland, Taylor raises more concerns about holding the Ukraine aid.

Intelligence community Inspector General Michael Atkinson notifies House Intelligence Chairman Adam Schiff and ranking member Rep. Devin Nunes of an "urgent concern" -- the whistleblower complaint -- that acting Director of National Intelligence Joseph Maguire has not allowed him to forward to Congress as required by law.

Three House committees launch an investigation of efforts by Trump, Giuliani and others to pressure the Ukrainian government to assist the President's reelection efforts and the committees request information about Trump's July 25 phone call with Zelensky.

September 11 -- White House lifts freeze on Ukraine aid

The White House lifted its temporary hold on $391 million in military and security assistance for Ukraine, according to testimony from key witnesses.

September 12 -- Ukraine informed hold on military aid is lifted

Taylor tells Zelensky that the hold on the military aid is lifted and says it is key to not get involved in foreign elections. An interview scheduled for the next day with CNN's Fareed Zakaria is canceled.

September 24 -- House Speaker Nancy Pelosi announces a formal impeachment inquiry into Trump

The day before the transcript of Trump's call with Zelensky is released and two days before the whistleblower complaint is made public, House Speaker Nancy Pelosi announces in a televised address that existing House oversight investigations into Trump will be expanded into an impeachment inquiry.

CORRECTION: This story has been updated to correctly reflect the date that the hold on Ukraine aid was lifted.