(Reuters) - Some Democrats are calling for the impeachment of U.S. President Donald Trump over reports that he asked his Ukrainian counterpart to launch an investigation that could damage his Democratic political rival Joe Biden.

FILE PHOTO: Democratic presidential candidate and former Vice President Joe Biden speaks at the One Iowa and GLAAD LGBTQ Presidential Forum in Cedar Rapids, Iowa, September 20, 2019. REUTERS/Scott Morgan

The Wall Street Journal and the New York Times on Friday reported that during a July phone call Trump repeatedly asked Ukrainian President Volodymyr Zelenskiy to investigate the involvement of Biden’s son, Hunter, with a Ukrainian energy company.

The phone call between the two world leaders was reportedly part of a whistleblower complaint against Trump from within the U.S. intelligence community.

The reports about the phone call also said Trump asked Zelenskiy to investigate whether Biden misused his position as vice president under former President Barack Obama to threaten to withhold U.S. aid unless a prosecutor who had investigated the energy company in which Biden’s son was involved was fired.

Biden, now a Democratic presidential hopeful, has confirmed he wanted the prosecutor fired. But no evidence has emerged that the former vice president did so to help his son. The wider U.S. government, the European Union and other international institutions also wanted the prosecutor fired because of an alleged failure to pursue major corruption cases.

The following timeline details key moments in the Biden family’s involvement in Ukrainian affairs and Trump’s effort to use those actions as political ammunition.

April 21, 2014: Biden, then vice president, visited Ukraine’s capital, Kiev, in a show of support for the country’s government amid rising tensions with Russia.

May 13, 2014: Burisma Group, a private company that has drilled for natural gas in Ukraine since 2002, announced that Hunter Biden would be joining its board. Around that time, Burisma’s founder, a former government official named Mykola Zlochevsky, was under investigation for alleged money laundering by Britain’s Serious Fraud Office.

Dec. 8, 2015: Biden visited Kiev again and spoke out against bureaucratic corruption that he said was eating Ukraine “like a cancer.” Biden threatened to withhold loan guarantees unless Ukraine’s top prosecutor, Viktor Shokin, who had been widely accused of corruption, was removed.

Shokin had investigated Burisma but the probe was dormant at the time Biden pushed for the prosecutor’s termination, Bloomberg reported earlier this year, citing a former Ukrainian official.

March 29, 2016: The Ukrainian Parliament voted to remove Shokin.

Jan. 23, 2018: Biden, at a Council on Foreign Relations event, detailed how he had threatened to withhold funds from Ukraine if Shokin was not removed.

April 2019: Hunter Biden’s term as a Burisma board member expired and he stepped down from the company.

May 16, 2019: Bloomberg quotes Ukraine’s current top prosecutor as saying he had no evidence of wrongdoing by Hunter Biden or his father.

May 11, 2019: Trump’s lawyer, Rudy Giuliani, said he was canceling a planned trip to Ukraine. Giuliani came under criticism from Democratic lawmakers after saying in media interviews that he had planned to meet with Ukraine’s new leaders and push for investigations that could help Trump.

July 25, 2019: Trump spoke with Zelenskiy and reportedly pressured him to investigate Hunter Biden.

Aug. 12, 2019: A whistleblower from within the intelligence community lodged a complaint with a internal watchdog that reportedly focused on Trump’s contacts with Ukraine.