House Democrats on Monday subpoenaed the heads of the Defense Department and Office of Management and Budget for documents related to the Trump administration's decision to withhold financial aid to Ukraine while the president pushed the allied nation for an investigation into former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE and his son, Hunter.

In letters to Defense Secretary Mark Esper Mark EsperOvernight Defense: Stopgap spending measure awaits Senate vote | Trump nominates former Nunes aide for intelligence community watchdog | Trump extends ban on racial discrimination training to contractors, military Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Official: Pentagon has started 'prudent planning' for full Afghanistan withdrawal by May MORE and Office of Management and Budget (OMB) acting director Russell Vought, the three committee chairmen leading the House's impeachment inquiry asked for the documents to be provided by Oct. 15.

“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,” House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.), Foreign Affairs Committee Chairman Eliot Engel Eliot Lance EngelHouse panel halts contempt proceedings against Pompeo after documents turned over Engel subpoenas US global media chief Michael Pack The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep MORE (D-N.Y.) and Oversight Committee Chairman Elijah Cummings Elijah Eugene CummingsBlack GOP candidate accuses Behar of wearing black face in heated interview Overnight Health Care: US won't join global coronavirus vaccine initiative | Federal panel lays out initial priorities for COVID-19 vaccine distribution | NIH panel: 'Insufficient data' to show treatment touted by Trump works House Oversight Democrats to subpoena AbbVie in drug pricing probe MORE (D-Md.) wrote.

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Congress had appropriated $250 million to the Defense Department for the Ukraine Security Assistance Initiative. But according to reports, President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE asked Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE, currently the acting White House chief of staff, to put a hold on the Ukraine aid in July 2019.

Trump held a call with Ukrainian President Volodymyr Zelensky on July 25 in which he urged an investigation into the Bidens and said he would instruct his personal lawyer, Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE, and Attorney General William Barr Bill BarrFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Why a backdoor to encrypted data is detrimental to cybersecurity and data integrity FBI official who worked with Mueller raised doubts about Russia investigation MORE to follow up with the Ukrainian government.

The White House eventually released its hold on the aid around the date of Sept. 11, following multiple inquiries from members of Congress about why the aid was delayed.

The committee chairmen are demanding that Esper hand over documents related to Trump's phone calls with the Ukrainian president and efforts by any current or former Trump administration officials to pressure the Ukrainian government to investigate the Bidens.

They are also seeking documents from both Esper and Vought on the delay of foreign assistance to Ukraine, including communications with people in the White House and other agencies, as well as the decision in September announcing that the aid would go forward. In addition, the committee chairmen are asking for documents related to communications with Congress about the status of the foreign aid, such as briefings and written notifications.

The House committee chairmen have issued three other subpoenas as part of the impeachment inquiry launched two weeks ago.

They have subpoenaed Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE, Giuliani and Mulvaney for documents related to Trump's conversations with Zelensky and the efforts to hold up the financial aid for Ukraine.

The committees also sent a letter on Friday to Vice President Pence asking for documents related to his role in Trump's efforts to press for an investigation into the Bidens, but the request was not a subpoena.

The chairmen reiterated a threat to other subpoena recipients in recent days that failing to comply with their demands would be used as evidence of obstruction in potentially writing articles of impeachment.

"Your failure or refusal to comply with the subpoena, including at the direction or behest of the president or the White House, shall constitute evidence of obstruction of the House's impeachment inquiry and may be used as an adverse inference against you and the president," they wrote in the letters to Esper and Vought.

The three House committees are also conducting depositions this week with two State Department officials. Ambassador to the European Union Gordon Sondland is scheduled to appear on Tuesday, while former Ukraine Ambassador Marie Yovanovitch is set to speak with lawmakers on Friday.

Updated at 1 p.m.