Volker is the first witness called by Republicans to testify publicly, appearing alongside former Trump National Security Council adviser Tim Morrison. But Volker, unlike nearly everyone else in the Ukraine saga, had a role in virtually every facet of the unfolding scandal.

He was a crucial point of contact for Rudy Giuliani, the president's personal lawyer, as Giuliani leaned on Ukrainians to launch investigations of former vice president Joe Biden and of debunked allegations that Ukraine — not Russia — hacked a Democratic Party server in 2016.

Volker was also in the loop on July 18, when word spread throughout the Trump administration that the president had placed a hold on nearly $400 million in military aid to Ukraine with virtually no explanation.

And Volker was in the room for crucial White House briefings with Ukrainian officials and another U.S. ambassador — Gordon Sondland — who later directly told top Ukrainians that military aid would likely hinge on the opening of Trump's favored investigations.

But Volker now says he was in the dark for many of the crucial moments that would have pointed toward a more nefarious purpose of the Ukraine maneuverings. He said that when he helped facilitate contact between Giuliani and a senior Ukrainian official, he was not aware that anyone considered a "linkage" between U.S. military aid and the investigations Trump sought.

"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 it 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."

Volker also said he was unaware that then-National Security Adviser John Bolton had "strong concerns" about the appropriateness of the investigations.

In perhaps one of the most glaring updates to his earlier testimony, Volker said that during a July 10 meeting at the White House with top Ukrainian officials, he now recalled that Sondland made a "generic comment about investigations" and that "all of us thought it was inappropriate."

In fact, Volker told lawmakers flatly during his closed-door deposition on Oct. 3 that investigations were not discussed at that meeting — testimony that was contradicted by other officials in the room. Rather, Volker said the meeting went poorly because the Ukrainians delivered a dry, bureaucratic presentation that didn't help give their American counterparts a clear picture of the political dynamic in Ukraine facing its new president, Volodymyr Zelensky.

Though several other witnesses also indicated that Bolton immediately shut down the meeting following Sondland's comment, Volker does not reference it even in his updated testimony. Volker also said he may have been having a "side conversation" in a subsequent meeting in the White House's Ward Room when other witnesses said the investigations were discussed further.

Throughout his original Oct. 3 testimony, Volker repeatedly drew a distinction between requests that Ukraine investigate Burisma — a Ukrainian energy company with a history of corruption — and Biden, whose son Hunter was tapped to sit on Burisma’s board as the company attempted to clean up its public image.

"At no time was I aware of or knowingly took part in an effort to urge Ukraine to investigate former Vice President Biden," Volker said. "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."

"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."

Volker repeated his earlier claim that he viewed connecting Giuliani with Ukrainians as a way to correct Giuliani's notion that the new Ukrainian president was vulnerable to being pulled into the country's long track-record of corruption.

"The Ukrainians believed that if they could get their own narrative across in a way that convinced Mayor Giuliani that they were serious about fighting corruption and advancing reform, Mayor Giuliani would convey that assessment to President Trump, thus correcting the previous, negative narrative," Volker said. "That made sense to me and I tried to be helpful."

