Kurt Volker, the former special envoy to Ukraine, testified in the impeachment inquiry on Tuesday afternoon, repeatedly remarking that allegations of corruption leveled at former Vice President Joe Biden are "not credible."

The ambassador, who was called to testify by Republicans, also sought to revise his testimony from his closed-door deposition. "I have learned many things that I did not know at the time of the events in question," he told congressional investigators today.

Volker was originally seen as a witness who would be sympathetic to the Republicans' case; other witnesses implicated him in efforts to strong-arm Ukrainian President Volodymyr Zelenskiy into publicly pursuing an investigation into Burisma Group and former Vice President Joe Biden, whose son Hunter sat on the board of the Ukrainian energy company.

But the former special envoy clarified on Tuesday that he was unaware of the investigative link between Biden and Burisma. A probe into the latter made sense, he said, while an investigation into the former did not.

"At no time was I aware of or knowingly took part in an effort to urge Ukraine to investigate former Vice President Biden," Volker testified. "I did not know that President Trump or others had raised Vice President Biden with the Ukrainians, or had conflated the investigation of possible Ukrainian corruption, with investigation of the former Vice President. In retrospect, for the Ukrainians, it clearly would have been confusing."

During Volker's closed-door testimony, he refuted the notion that the investigations were mentioned on a July 10 phone call with Ukraine. But on Tuesday, he reversed course, explaining that during the conversation, Gordon Sondland, the U.S. ambassador to the European Union, made a "generic comment about investigations" that he thought was "inappropriate." Volker's revision comports with the deposition of Lt. Col. Alexander Vindman, who said that former National Security Adviser John Bolton reprimanded Sondland for the remarks.

Trump and his associates have accused Joe Biden of moving to depose a prosecutor in Ukraine to protect his son from a potential investigation related to his association with Burisma. Multiple international agencies sought the prosecutor's removal on charges of corruption, and the former vice president maintains he was acting on President Obama's foreign policy directive. George Kent, the deputy assistant secretary of state for European and Eurasian affairs, previously testified that there was no truth "whatsoever" to Trump's accusations.

Over the summer, the president froze $400 million in military aid to Ukraine without explanation. That aid was not disbursed until September 11.

"I opposed the hold on U.S. security assistance as soon as I learned about it on July 18, and thought we could turn it around before the Ukrainians ever knew or became alarmed about it," Volker testified on Tuesday. "I did not know the reason for the hold, but I viewed it as a U.S. policy problem that we needed to fix internally, and I was confident we would do so."