In 2014, the then US vice president Joe Biden was at the forefront of American diplomatic efforts to support Ukraine’s fragile democratic government as it sought to fend off Russian aggression and root out corruption. So it raised eyebrows when Biden’s son Hunter was hired by a Ukrainian gas company.

The Obama White House said at the time that there was no conflict because the younger Biden was a private citizen. And there’s been no evidence of wrongdoing by either Biden.

Yet the matter is back in the spotlight following revelations that President Donald Trump prodded Ukraine’s president to help him investigate any corruption related to Joe Biden, now one of the top Democrats seeking to defeat Trump in 2020. Trump’s private lawyer, Rudy Giuliani, has also publicly urged Ukrainian officials to investigate the Bidens.

Hunter Biden was named a paid board member of Burisma Holdings in April 2014. The company’s founder was a political ally of Viktor Yanukovych, Ukraine’s Russia-friendly president who was driven out in February 2014 by mass protests.

Yanukovych’s ouster prompted the Obama administration to move quickly to deepen ties with Ukraine’s new government. Joe Biden played a leading role, traveling to Ukraine and speaking frequently with its new Western-friendly president.

The younger Biden’s business role raised concerns among anticorruption advocates that Burisma was seeking to gain influence with the Obama administration. At the time, the company ran a natural gas extraction operation in Crimea, a Ukrainian peninsula annexed by Russia after Yanukovych was pushed from power.

Hunter Biden has denied using his influence with his father to aid Burisma. He remained on the board through early 2019, often appearing at energy-related conferences abroad representing Burisma’s interests.

On Saturday, the former vice president said he never speaks to his son about his overseas business dealings.

The matter, however, has continued to be questioned by Trump and his allies. They’ve pointed in particular to Biden’s move in March 2016 to pressure the Ukrainian government to fire its top prosecutor, Viktor Shokin, who had previously led an investigation into Burisma’s owner.

Biden’s was representing the official position of the US government, a position that was also supported by other Western governments and many in Ukraine, who accused Shokin of being soft on corruption.

Corruption has continued to fester in Ukraine. In May, the country’s new president, Volodymyr Zelenskiy, came into office with no political experience but with bold promises to put an end to the corrupt practices.

Around this time, Giuliani began reaching out to Zelenskiy and his aides to press for a government investigation into Burisma and Hunter Biden’s role with the company.

In a Fox News interview on May 19, Trump claimed the former Ukrainian prosecutor “was after” Joe Biden’s son and that was why the former vice president demanded he be fired. There is no evidence of this.

Ukraine’s current prosecutor, Yuriy Lutsenko, was quoted by Bloomberg News in May as saying he had no evidence of wrongdoing by Biden or his son. Bloomberg also reported that the investigation into Burisma was dormant at the time Biden pressed for Shokhin’s ouster.