House Democrats on Friday demanded that Vice President Mike Pence turn over documents related to President Donald Trump's call with the Ukrainian president at the heart of their rapidly unfolding impeachment probe.

The request for documents was made in a letter sent to the vice president and provided a deadline of Oct. 15. It was signed by the chairmen of the Foreign Affairs, Intelligence and Oversight committees.

The letter follows a report published in The Washington Post on Tuesday that revealed that Pence's national security advisor was on the July 25 call between Trump and Ukrainian President Volodymyr Zelensky that sparked the impeachment inquiry.

Trump urged Zelensky on the call to investigate his 2020 rival Joe Biden, the former vice president. On Thursday, CNN reported that Pence was provided a transcript of the phone call the day after it took place. A source told CNN that Pence "may or may have not read it."

Citing the public reporting, the committee chairmen wrote in the letter that it "remains unclear to what extent you had knowledge of the specific aspects of some of these events."

Also under scrutiny is Pence's Sept. 1 meeting with Zelensky in Warsaw, Poland. According to the Post report, Trump instructed Pence to inform Zelensky that Washington would not release military aid Congress had allocated to the country unless the leader committed to fighting corruption.

In a statement, Pence's press secretary Katie Waldman said that the vice president received the letter after it was released to the media.

"Given the scope, it does not appear to be a serious request but just another attempt by the Do Nothing Democrats to call attention to their partisan impeachment," Waldman said. "Despite their efforts to distract and obstruct our agenda, this Administration will stay focused on creating jobs, securing the border, rebuilding our military, negotiating better trade deals, and the issues most important to the American people."

The committees requested a broad range of documents related to Trump's July 25 call with Zelensky, as well as another call, which took place in April.

The Democrats asked for all "recordings, transcripts, notes," of the calls as well as the names of all the people who were on the calls or who had a role in transcribing them. They also requested all communications related to the calls between current or formal officials in the White House, the Department of Justice, the Department of State, the Department of Energy, and the various intelligence agencies.

The committee heads also asked Pence to turn over documents related to his meeting with Zelensky in September, including any communications about the meeting with Energy Secretary Rick Perry, White House senior advisor Jared Kushner and others.

The committee chairmen warned Pence in the letter that failure to turn over the documents they requested "shall constitute evidence of obstruction" in the impeachment inquiry.

The impeachment inquiry has moved at a brisk pace since House Speaker Nancy Pelosi announced her support for it last month. The three committees have subpoenaed Secretary of State Mike Pompeo and Trump's personal attorney Rudy Giuliani for documents related to the probe and have threatened to issue a subpoena to the White House if it fails to comply with their requests.