Vice President Pence has refused to declassify testimony that is “directly relevant” to the impeachment debate, House Intelligence Committee Chairman Adam B. Schiff (D-Calif.) wrote Tuesday in a letter that raised further questions about what Pence said in a September phone call with Ukrainian President Volodymyr Zelensky.

In a letter to Pence, Schiff wrote that classified witness testimony gathered during the impeachment inquiry “raises profound questions about your knowledge of the President’s scheme to solicit Ukraine’s interference in the 2020 U.S. presidential election.”

Pence spokeswoman Katie Waldman declined to comment.

The testimony from Jennifer Williams, Pence’s Russia adviser, was provided as a supplemental written submission to the Intelligence Committee through her lawyer Nov. 26. Ten days later, Schiff asked Pence to declassify it. But what she said about Pence’s Sept. 18 call with Zelensky has remained out of public view.

Schiff wrote that in a letter last week, Pence’s office refused to declassify Williams’s testimony. In the letter, Pence’s office also denied that the vice president discussed with Zelensky specific topics that President Trump raised two months earlier in a separate call with the Ukrainian leader.

“Without prompting, the letter volunteers that ‘the Vice President never raised the Bidens, Burisma, or Crowdstrike in his conversations with President Zelensky,’ ” Schiff’s letter stated. “The Committee neither asserted that, nor asked whether, you specifically used those words.”

Schiff called the letter from Pence’s office “deeply troubling,” writing that if Williams’s testimony is true, it suggests the vice president had knowledge about the efforts to pressure Ukraine to investigate Trump’s political opponents. He did not specify what in her testimony potentially implicated Pence.

“If the Supplemental Submission [from Williams] accurately describes your September 18 conversation with President Zelensky . . . it would mean that the representation of your communications with President Zelensky, as described in your office’s December 11 letter, may be purposefully misleading.”

Pence and his office have sent mixed messages about the need for the details of the call to remain secret. The vice president said Nov. 6 that he had “no objection at all” to releasing records of his calls with Zelensky and added that his office was “working with White House counsel about that.” Yet Williams’s lawyer said Nov. 19 that Pence’s office had declared the call “classified.”

Schiff urged Pence on Tuesday to reconsider the decision not to declassify Williams’s written testimony.

“Your unwillingness to declassify the Supplemental Submission raises the serious question of whether your continuing efforts to obstruct the House’s impeachment inquiry are intended not just to protect President Trump, but yourself as well,” Schiff wrote.

Few details are known about the Sept. 18 call.

That day, the White House released a readout saying that Pence and Zelensky discussed the latter’s upcoming meeting with Trump at the United Nations, the U.S. commitment to Ukraine’s sovereignty and Zelensky’s “bold action to tackle corruption through legislative reforms.”

During her Nov. 7 deposition, Williams said the purpose of the call was to follow up on a “successful” Sept. 1 meeting between Zelensky and Pence and to “reiterate the news that the security assistance hold had been lifted.”

Williams described the call as “very positive” and said Pence had asked “a bit more about, you know, how Zelensky’s efforts were going.” She said the call included no mention of investigations, vice president Joe Biden, the Ukrainian gas company Burisma or the cybersecurity company CrowdStrike.

Schiff’s interest in the episode has been clear since at least Nov. 19, when Williams appeared at a public hearing and Schiff referred to the call during his first question.

“Is there something about that call that you think may be relevant to our investigation?” he asked, after Williams confirmed she had listened and taken notes.

Williams’s lawyer responded instead, saying that because Pence’s office had taken the position that the call was classified, he would refer lawmakers to Williams’s Nov. 7 deposition answers and the public readout of the call.

Schiff then asked: “Ms. Williams, I would only ask you in this setting whether you think there is something relevant to our inquiry in that call and whether, if so, you’ll be willing to make a classified submission to the committee?”

Williams referred to her previous testimony and said she would be “very happy to appear for a classified setting discussion as well.”

Schiff said it wouldn’t be necessary for her to appear if she made a classified submission in writing. “I’ll be happy to do so,” Williams said.