US President Donald Trump (L) and US Vice President Mike Pence at Joint Base Myer-Henderson Hall, Virginia.Brendan Smialowski | AFP | Getty ImagesHouse 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 committees on Foreign Affairs, Intelligence and Oversight. 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.Citing the Post report, 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.” The committees requested all documents related to Trump’s April 21 and July 25 phone calls with Zelensky, communications between administration officials about the calls, and efforts by Trump’s personal attorney Rudy Giuliani or members of the administration to press Ukrainian government officials to investigate Biden and his son Hunter. 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. This is breaking news. Check back for updates.