Vice President Mike Pence's national security adviser Keith Kellogg was on the July 25 phone call between President Trump and Ukrainian President Volodymyr Zelensky that is now at the heart of House Democrats' impeachment inquiry, the Washington Post reports, citing "current and former U.S. officials."

Why it matters: Pence likely would have been briefed the following day on the details of the phone call, in which Trump asked Zelensky to investigate Joe Biden. The summary of the call, which has since been released by the White House, also likely would have been included in Pence's briefing materials ahead of a Sept. 1 trip to Warsaw in which he met with Zelensky and informed him that the administration had frozen millions of dollars in aid to Ukraine.

The big picture: Officials tell the Post that neither Pence nor Kellogg were aware that the call had set off alarm bells within the White House, which eventually led to an official whistleblower complaint being filed with the intelligence community inspector general. They also claim that Pence was not aware of Trump's and Rudy Giuliani's broader efforts to push Ukraine to investigate Biden.

However, Trump did instruct Pence not to attend Zelensky's inauguration in May and to later break the news to the Ukrainian president that the aid would not be released until he took more aggressive action on corruption.

Former officials tell the Post that the vice president's emphasis on corruption likely would have been interpreted by the Ukrainians as "code" for the Biden issue, but Pence chief of staff Marc Short insists that this was not the case.

Short points to the fact that the aid to Ukraine was eventually released after the meeting. An official also told the Post that Pence informed Trump that Zelensky had a "good heart" and urged him to release the aid.

Between the lines: The Post contends that Trump's reliance on Pence to convey messages to Zelensky is part of a broader strategy by the president of using administration officials to "advance his personal or political interests — even in cases when those subordinates appear not to know that another agenda is in play."

Go deeper: What Joe and Hunter Biden actually did in Ukraine