Did Trump improperly pressure Ukrainian president Volodymyr Zelensky to investigate his political rival, Joe Biden, and his son, Hunter, as Democrats contend? A timeline of events that led to the release of a five-page summary of Trump and Zelensky's conversation.

July 24

Special counsel Robert Mueller testifies before two House committees about Russia's "sweeping and systematic" interference in the 2016 election. Mueller found numerous contacts between Russians and Donald Trump's presidential campaign but no conspiracy to influence election results.

July 25

President Trump calls Ukrainian President Volodymyr Zelensky, urging him to fight corruption and suggesting he investigate Democratic rival Joe Biden and his son Hunter, who had business interests in Ukraine when his father was vice president and working closely with the Ukrainian government.

Aug. 12

A whistleblower reports an “urgent concern” to the inspector general for national intelligence, Michael Atkinson. The New York Times and The Washington Post say the Ukraine call and whistleblower complaint are related.

Aug. 26

After preliminary review, the inspector general finds the complaint credible and reports it to the acting director of national intelligence, Joseph Maguire.

Sept. 9

The inspector general reports the complaint without its substance to House Intelligence Chairman Adam Schiff, D-Calif. The inspector general says Maguire disagrees that the complaint merits “urgent concern,” which would have required reporting to congressional intelligence committees within seven days.

Three House chairmen – Schiff; Foreign Affairs Chairman Eliot Engel, D-N.Y.; and Oversight and Reform Chairman Elijah Cummings, D-Md. – announce a wide-ranging investigation into the dealings with Ukraine by Trump and his personal lawyer Rudy Giuliani.

Sept. 17

The inspector general tells Schiff that Maguire and the Justice Department have told him not to report the substance of the complaint to Congress.

Sept. 19

The House Intelligence Committee meets behind closed doors with Atkinson, who doesn’t disclose details about the complaint.

Sept. 24

House Speaker Nancy Pelosi, D-Calif., announces that the House will begin a formal impeachment inquiry into Trump – the culmination of a months-long investigative battle between Democrats and the White House that came to a head over the president's efforts to press Zelensky to investigate the Bidens.

Sept. 25

The Department of Justice released a five-page summary of the 30-minute conversation between Trump and Zelensky.

Sept. 26

The whistleblower's complaint that has sparked an impeachment inquiry against President Donald Trump is released, revealing deep concern that the president "used the power of his office" to solicit foreign help to discredit one of his main political rivals during a July telephone call with the president of Ukraine.

The whistleblower, who has not been identified publicly, went on to detail efforts by senior White House officials to later "lock down" access to all records of the July 25 call with Ukrainian President Volodymyr Zelensky in which Trump urged his counterpart to investigate Democratic presidential candidate Joe Biden.

The acting director of national intelligence, Joseph Maguire, told the House Intelligence Committee that he delayed a whistleblower complaint from being passed along to Congress because of executive privilege to protect communications with President Donald Trump.

Oct. 3

House Democrats released text messages that further fueled speculation that the White House dangled military aid and a meeting with President Donald Trump as part of an effort to get Ukraine to investigate political rival Joe Biden.

The messages between Trump's then-special envoy to Ukraine, Kurt Volker, and other U.S. diplomats were sent before and after Trump asked Ukrainian President Volodymyr Zelensky in a July 25 phone call for "a favor" to investigate Biden and his son, Hunter, on charges of corruption.

The documents describe text messages exchanges between five people:

Kurt Volker: former U.S special envoy for Ukraine

Bill Taylor: the top American diplomat in Ukraine

Gordon Sondland: U.S. Ambassador to the European Union

Rudy Giuliani: Trump's personal lawyer

Andrey Yermak: advisor to Ukranian President Volodymyr Zelensky

An excerpt from September 9:

Read a breakdown of some of the most important messages that Democrats released here.

Oct. 6

A second whistleblower "in connection to" the allegations surrounding the call between President Donald Trump and Ukrainian President Volodymyr Zelensky will be coming forward, according to that person's attorney.

"I can confirm that my firm and my team represent multiple whistleblowers in connection to the underlying August 12, 2019, disclosure to the Intelligence Community Inspector General," tweeted attorney Andrew Bajak, whose firm Compass Rose also represents the first whistleblower.

ABC News reported that the second whistleblower – "described as an intelligence official – has firsthand knowledge of some of the allegations outlined in the original complaint."

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