An adviser to Vice President Mike Pence told House impeachment investigators this month that President Trump's request for Ukraine President Volodymyr Zelensky to open an investigation into former Vice President Joe Biden and his family "struck me as unusual and inappropriate" and "shed some light on possible other motivations behind a security assistance hold."

The three House committees overseeing the impeachment inquiry against Trump released hundreds of pages of closed-door testimony by Jennifer Williams, a special adviser to Pence on Europe and Russia, and Tim Morrison, the former senior director for European affairs at the National Security Council. Morrison was interviewed Oct. 31 and Williams was interviewed Nov. 7.

Both Williams and Morrison were listening in on the July 25 phone call, which was flagged by an intelligence committee whistleblower and has become the center of the impeachment inquiry. House Democrats say that aid was withheld from Ukraine until its government launched investigations into Biden, his son Hunter and their dealings in the eastern European county -- particularly the younger Biden's work with Ukrainian natural gas company Burisma Holdings.

Investigators also met Saturday with Office of Management and Budget (OMB) official Mark Sandy, who was involved in key meetings about the nearly $400 million aid package Congress had approved for Ukraine.

Williams told lawmakers that the word "Burisma" appeared in her notes of the July 25 call, though it did not appear in the memorandum of the conversation released by the White House in September. The transcript indicates that Williams initially told lawmakers that Trump mentioned Burisma to Zelensky, but a letter from her attorney following the deposition states that Williams' "recollection had been incorrect" and Zelensky mentioned Burisma to Trump.

For his part, Morrison that he consulted NSC lawyers following the July 25 call because he was worried about details of the conversation being leaked to the media, according to transcripts of his testimony released Saturday.

In his testimony, Morrison said he asked NSC Legal Adviser John Eisenberg and Eisenberg's deputy, Michael Ellis, to review the July 25 call "because I was concerned about whether or not they would agree that it would be damaging ... if the call package, if the call mem-con [memorandum of conversation] or its contents leaked."

During the call itself, Morrison said he was concerned by how "obsequious" Zelensky sounded toward Trump and added that the conversation "was not the full-throated endorsement of the Ukraine reform agenda that I was hoping to hear."

Morrison added that he and Eisenberg agreed that access to the call should be restricted, but then claimed Eisbenberg later told him that he did not intend for the call summary to be placed on a highly classified server. According to Morrison, Eisenberg's staff apparently put it there by mistake and nothing in the call's contents warranted placement on the classified server.

Morrison also told House investigators that Fiona Hill, a top NSC expert on Russia, had described U.S. Ambassador to the European Union Gordon Sondland "as a problem" who "believed he had the mandate to get involved [in Ukraine policy] based on his relationship with" Trump, even though Ukraine is not a member of the EU.

"We [Morrison and Hill] both discussed that Ukraine was not in the EU, which led to the follow-on question of, why is he involved in Ukraine?" said Morrison of Sondland, who is expected to testify publicly on Wednesday. Morrison, who is set to publicly testify Tuesday, added that he became concerned while listening to the July 25 phone call that Hill's fear that Sondland was involved in a "parallel process" of making Ukraine policy was true. Read more at FOX News