House Democrats on Monday fired off a new round of subpoenas in their impeachment inquiry, requiring the Defense Department and the Office of Management and Budget to turn over documents related to President Donald Trump's decision to temporarily withhold military aid for Ukraine.

The House Intelligence Committee sent a letter to Defense Secretary Mark Esper and Office of Management and Budget Acting Director Russell Vought, compelling them to produce the requested documents by Oct. 15. Esper recently said he was not part of the July 25 call between Trump and Ukrainian President Volodymyr Zelenskiy that's under major scrutiny.

The impeachment committees have ramped up their investigation centering on a whistleblower complaint about Trump's July 25 call where he asked Zelenskiy to look into former Vice President Joe Biden and his son Hunter Biden. House Democrats have also subpoenaed the Trump administration, Secretary of State Mike Pompeo and Trump's personal lawyer Rudy Giuliani for similar documents. They've also requested documents from Vice President Mike Pence.

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The Intelligence Committee specifically wants to look into why Trump froze Ukraine's military aid that was already appropriated in 2018 to help fight Russian aggression against the region. Monday's letter points to reports that Trump in July asked acting chief of staff and OMB Director Mick Mulvaney to halt that assistance. According to multiple reports, White House officials were directed to tell lawmakers that delays in the aid were part of an "interagency process."

Trump has denied claims of quid pro quo – using the funding as leverage in order to get Ukraine to open a corruption investigation into the Bidens – and has said that he froze the money because he wants other European countries to pay their fair share to help the country.