Kurt Volker, the former U.S. envoy to Ukraine, significantly revised his closed-door deposition during the public impeachment hearings Tuesday.

In his opening statement, Volker asserted multiple times that when we testified on Oct. 3, he was not fully informed.

Volker initially said that President Donald Trump‘s desired political investigations and the release of about $400 million in US military aid to Ukraine were not mentioned in a July 10 meeting at the White House. On Tuesday, Volker said he now knows that the investigations were mentioned.

When questioned by Adam Schiff (D-California) why he did not disclose the meeting in his original deposition, Volker said he did not recall it at the time.

SLIDESHOW: TOP DEMOCRATS RUNNING FOR PRESIDENT IN 2020

“I read the account by Alex [Vindman] and that jogged my memory,” he said.

Volker’s changes have raised eyebrows, prompting some to question whether he committed perjury. His defense is that he was uninformed at the time of the first testimony and recent events have caused him to amend what he said previously.

“I have learned many things that I did not know at the time of the events in question,” Volker told the House Intelligence Committee Tuesday.

He was the first witness called by Republicans to testify publicly, along with former National Security Council adviser Tim Morrison.

Volker has also reversed the claim from his original testimony that the idea of investigating Biden and a conspiracy theory about the 2016 election never came up in a White House meeting two weeks prior to the July 25 phone with Ukraine that sparked a whistleblower complaint and the subsequent impeachment inquiry.

He said he did not correlate investigating Burisma Holdings, a Ukraine-based natural gas company with investigating Biden. Biden’s son, Hunter Biden, sat on the board of Burisma at the time.

“In hindsight, I now understand that others saw the idea of investigating possible corruption involving the Ukrainian company, ‘Burisma,’ as equivalent to investigating former Vice President Biden,” Volker added. “I saw them as very different — the former being appropriate and unremarkable, the latter being unacceptable.”

He also criticized Trump’s baseless theory that Joe Biden used the power of the Vice Presidency to stop an investigation into Burisma.

“I have known former Vice President Biden for 24 years, and the suggestion that he would be influenced in his duties as Vice President by money for his son simply has no credibility to me,” Volker said Tuesday. “I know him as a man of integrity and dedication to our country.”

The GOP-picked witness continued to defend the democratic frontrunner saying, “I also said at that July 19 meeting that it is not credible to me that former Vice President Biden would have been influenced in anyway by financial or personal motives in carrying out his duties as Vice President.”

Volker said Trump’s request to look into potential Ukrainian interference in the 2016 election is a “different issue.” The theory has been completely discredited by numerous studies.

“A different issue is whether some Ukrainians may have attempted to influence the 2016 election or thought they could buy influence: that is at least plausible, given Ukraine’s reputation for corruption,” Volker said. “But the accusation that Vice President Biden acted inappropriately did not seem at all credible to me.”

The other testimonies scheduled for this week include U.S. ambassador to the European Union Gordon Sondland, deputy assistance secretary of Defense Laura Cooper and David Hale, the undersecretary of State, who will all testify on Wednesday. On Thursday, Fiona Hill, the former senior director for Europe and Russia at the NSC, and David Holmes, a political counselor at the U.S. Embassy in Kiev will testify.

The slew of testimonies indicate that the Democrats are on track to impeach the president by Christmas.