Former Ambassador Marie Yovanovitch, one of the star witnesses of the Democrats’ impeachment inquiry, on Friday undercut the defense Joe Biden and his allies have been making about the firing of Ukraine’s top prosecutor.

Yovanovitch, a self-described anti-corruption expert who most recently served as U.S. ambassador to Ukraine between 2016-2019, told the House Intelligence Committee there was an open probe into Burisma Holdings when Joe Biden demanded the firing.

Burisma, which is Ukraine’s only private oil conglomerate and until recently counted Hunter Biden among its board of directors, is central to the impeachment inquiry into President Donald Trump. Democrats have argued Trump’s suggestion that the Ukrainian government investigate the company and its ties to the younger Biden amount to an impeachable offense.

During Friday’s hearing, Yovanovitch was questioned about the allegations of public corruption that have dogged Burisma and its leadership since the early-2010s. In particular, she was asked about the status of a corruption probe Ukraine had into Burisma when Joe Biden pushed for the ouster of Viktor Shokin, the country’s prosecutor general, in 2016.

“It wasn’t an active case, but it also was not fully closed,” the former ambassador testified, before elaborating that at the time the Ukrainian government was opting to keep a “hook” into Burisma and its founder, Mykola Zlochevsky.

Yovanovitch’s admission that Shokin was looking for a “hook” with which to prosecute Burisma and Zlocvesky undermines a central argument Joe Biden has made to defend his conduct. Since the story became central to the impeachment inquiry, the former vice president and his allies have tried to argue that Shokin’s investigation into Burisma was dormant at the time of his firing and therefore his ouster actually improved the chances that the company would face stricter scrutiny.

Former Obama administration speechwriter Jon Favreu tweeted on the first day impeachment inquiry:

A reminder that Joe Biden’s push to get the corrupt Ukrainian prosecutor fired, which was the position of the U.S. government and basically the entire global community, was a move that INCREASED the chances that Burisma would be investigated.

A reminder that Joe Biden’s push to get the corrupt Ukrainian prosecutor fired, which was the position of the U.S. government and basically the entire global community, was a move that INCREASED the chances that Burisma would be investigated. — Jon Favreau (@jonfavs) November 13, 2019

The argument has taken on more importance as revelations continue to mount about potential conflicts of interest between Joe Biden’s political influence and his youngest son Hunter’s wheeling and dealing overseas.

Specifically father and son have struggled to explain the timing of Hunter Biden’s ascension to Burisma’s board of directors. The younger Biden secured the appointment in 2014 around the same time his father was appointed to oversee Obama administration policy in Ukraine.

As Peter Schweizer, senior contributor at Breitbart News, detailed in his book Secret Empires: How the American Political Class Hides Corruption and Enriches Family and Friends, Hunter Biden secured the position, which paid as much as $83,000 per month, despite having no background in energy or Ukraine.

Adding to concerns is the fact that at the time Hunter Biden joined Burisma, the company was seen as actively courting Western leaders to prevent further scrutiny of its business practices. The same month Hunter Biden was tapped for the group’s board, the government of Great Britain froze accounts belonging to Burisma founder Mykola Zlochevsky under suspicion of money laundering. Not only had Zlochevsky had his assets frozen in Great Britain, but the former Ukrainian minister of natural resources was also facing suspicions of public corruption at home. Zlochevsky would later be charged with corruption for using his ministerial office to approve oil and gas licenses to companies under his control.

A Ukrainian official with strong ties to Zlochevsky admitted in October the only reason that Hunter Biden secured the appointment was to “protect” the company from foreign scrutiny. The claim has credence given that at the time Joe Biden, as the sitting vice president, was tasked with leading the Obama administration’s policy towards Ukraine in response to Russia’s invasion of Crimea.

It is in the context of Burisma and Zlochevsky’s legal troubles that Joe Biden’s political influence has raised the most red flags. The former vice president has particularly drawn questions over his conduct in demanding the firing of Shokin, who was leading the probe into Burisma.

Joe Biden, who has publicly bragged about the firing, reportedly threatened to withhold more than one billion dollars in U.S. aid if the Ukrainian government did not remove Shokin. He has claimed the demand came from then-President Barack Obama, who had allegedly lost faith in the prosecutor’s ability to tackle corruption.

Unofficially, though, it was known that Shokin was investigating both Burisma and Zlochevsky for public corruption. It is uncertain if the probe extended to Hunter Biden, although Shokin has recently admitted that prior to his ouster he was warned to back off the matter. Regardless of what occurred, Shokin’s successor, who is now himself being investigated for public corruption, dropped the investigation into Burisma and Zlochevsky in 2017.

On Friday, Yovanovitch admitted during her testimony that convoluted history and the conflicts of interest between the former vice president and his youngest son were topics of concern for Obama administration officials during her confirmation as ambassador to Ukraine in 2016.