The chairmen of three powerful House committees on Monday issued subpoenas to the Pentagon and the White House Office of Management and Budget for documents relating to President Trump’s July phone call with Ukrainian President Volodymyr Zelensky, in the latest escalation of Democrats' impeachment inquiry.

House Intelligence Committee Chairman Adam Schiff, D-Calif., House Oversight and Reform Committee Chairman Elijah Cummings, D-Md., and House Foreign Affairs Committee Chairman Eliot Engel, D-N.Y., sent individual letters to Department of Defense Secretary Mark Esper and OMB Acting Director Russell Vought concerning reports that Trump ordered then-acting OMB Chief Mick Mulvaney to freeze military aid to Ukraine.

Mulvaney reportedly conveyed Trump’s order “through the budget office to the Pentagon and the State Department, which were told only that the administration was looking at whether the spending was necessary.” White House officials have denied any "quid pro quo" linking that aid to Trump's request for an investigation into the dealings of former Vice President Joe Biden and his son, Hunter, with Ukraine. The latest subpoenas aim to dig into the question.

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“The enclosed subpoena demands documents that are necessary for the Committees to examine this sequence of these events and the reasons behind the White House’s decision to withhold critical military assistance to Ukraine that was appropriated by Congress to counter Russian aggression,” the three Democratic lawmakers wrote in their letters. The Pentagon and OMB were given an Oct. 15 deadline to hand over the documents.

The subpoenas issued to the Pentagon and OMB follows a similar subpoena issued late Friday by the three lawmakers to the White House, ordering Mulvaney – now Trump’s acting chief of staff – to hand over documents relating to the phone call by Oct. 18.

Earlier Friday, House Democrats also sent a letter requesting documents by Oct. 15 from Vice President Mike Pence as part of the impeachment inquiry.

The inquiry is looking into whether Trump pressured Ukraine to interfere in the 2020 election by freezing military aid, then urging Zelensky to investigate former Biden, his son Hunter, and their business dealings in the Eastern European country. Democrats have claimed that the call revealed a quid pro quo in which Trump tied the request to nearly $400 million in frozen military money.

Trump has not denied that he asked the Ukrainian leader to investigate the Bidens, but denies that there was any move to withhold aid to the country if prosecutors in Ukraine did not look into his 2020 election rival.

Despite Trump’s denials, Democrats on Thursday released text messages between Andrey Yermak, an aide to Zelensky, and Kurt Volker, a former U.S. State Department special envoy to Kiev, which Democrats argue bolsters their claims against Trump.

Volker, in a text message on the morning of a planned July 25 phone call between Trump and Zelenskiy, wrote: "Heard from White House -- Assuming President Z convinces trump he will investigate / 'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington."

But the planning started to unravel when Zelensky's aide tried to lock in a date for the Trump meeting before putting out the statement on the investigations. Trump put a hold on military assistance to the country, which was depending on the funds as part of its defense of Russian separatists.

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Just over a week after Yermak's message to Volker about funding, Gordon Sondland, the U.S. ambassador to the European Union, and Bill Taylor, the top U.S. diplomat to Ukraine, texted each other. They were discussing the White House's decision to cancel Trump's meeting with Zelensky in Poland.

"As I said over the phone, I think it's crazy to withhold security assistance for help with a political campaign," Taylor wrote.

"Bill, I believe you are incorrect about President Trump's intentions," Sondland wrote back. "The President has been crystal clear: no quid pro quo’s of any kind. The President is trying to evaluate whether Ukraine is truly going to adopt the transparency and reforms that President Zelenskiy promised during his campaign.”

Sondland then recommended stopping the text exchange.

Fox News’ Alex Pappas, Samuel Chamberlain and Edmund Demarche and The Associated Press contributed to this report.