WASHINGTON — House Democrats on Monday issued a pair of subpoenas to Defense Secretary Mark Esper and Office of Management and Budget Acting Director Russell Vought demanding documents and communications regarding President Donald Trump’s decision to suspend U.S. aid to Ukraine.

Intelligence Committee Chairman Adam Schiff, D-Calif., issued the subpoenas as part of Democrats’ impeachment inquiry in consultation with House Oversight Committee Chairman Elijah Cummings, D-Md., and House Foreign Affairs Committee Chairman Eliot Engel, D-N.Y.

House Intelligence Committee Chairman Adam Schiff, along with two other House chairmen, issued subpoenas Monday for material related to the president's suspension of aid to Ukraine. Tom Brenner / Getty Images

"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 chairmen wrote in letters to each of the two top administration officials.

Under the subpoenas, Esper and Vought are required to produce requested documents to the Intelligence committee by Oct. 15.

The top Democrats alluded to news reports that claimed Trump ordered his acting chief of staff, Mick Mulvaney in July to freeze military assistance to Ukraine and that Mulvaney conveyed that order to the Pentagon’s budget office and the State Department. They also pointed to a New York Times report that said Pentagon officials "tried to make a case to the White House that the Ukraine aid was effective and should not be looked at in the same manner as other aid."

House Democrats launched the impeachment inquiry last month amid reports of Trump's July phone conversation with Ukrainian President Volodymyr Zelenskiy, during which he asked Zelenskiy to "look into" former Vice President Joe Biden and his son Hunter. Trump's decision to unexpectedly freeze nearly $400 million in military aid to the Ukraine days before the call has led to allegations that he was attempting a quid pro quo arrangement.

In early September, the committees first requested documents related to the suspension of aid and, after multiple follow-up requests, Oversight Chairman Cummings issued a subpoena Friday to Mulvaney for the documents. The aid was reinstated in September.

So far, Democrats have issued six subpoenas as part of the impeachment probe. The others target Secretary of State Mike Pompeo and Trump’s personal lawyer Rudy Giuliani. Democrats have also requested documents from Vice President Mike Pence.

Democrats plan to hear from U.S. Ambassador to the European Union Gordon Sondland behind closed doors on Tuesday about Ukraine and from former U.S. Ambassador to Ukraine Marie Yovanovitch on Friday.

Last week, former U.S. special envoy Kurt Volker and intelligence community inspector general Michael Atkinson testified before lawmakers about the initial whistleblower’s complaint.