WASHINGTON —House investigators subpoenaed documents from the Department of Defense and Office of Management and Budget on Monday about the withholding of military aid to Ukraine as the Democratic-led impeachment inquiry barreled forward.

Three House panels key to the impeachment inquiry said Monday that they had sent subpoenas to Defense Secretary Mark Esper as well as the Office of Management and Budget acting director Russell Vought for documents related to the withholding of U.S. military aid from Ukraine.

"The Committees are investigating the extent to which President Trump jeopardized national security by pressing Ukraine to interfere with our 2020 election and by withholding military assistance provided by Congress to help Ukraine counter Russian aggression, as well as any efforts to cover up these matters," the lawmakers said in a letter.

Signing the letter were Intelligence Committee Chairman Adam Schiff, D-Calif., Oversight Chairman Elijah Cummings, D-Md., and Foreign Affairs Chairman Eliot Engel, D-N.Y.

Trump told his acting chief of staff, Mick Mulvaney, to hold back almost $400 million in military aid for Ukraine shortly before his July call with Ukraine President Volodymyr Zelensky in which he pressed Zelensky to investigate former vice president Joe Biden and his son, Hunter Biden.


Trump has repeatedly denied that there was a "quid pro quo" between the military assistance and the request to investigate the Bidens.

On Sunday, the attorney representing the whistleblower who first raised an alarm about the July call said that "multiple" whistleblowers have come forward. Trump has repeatedly accused the first whistleblower of being "partisan" without citing evidence and claimed the whistleblower was "very wrong" about the July call.

House Democrats eager to protect the whistleblower are considering testimony at a remote location and possibly obscuring the individual's appearance and voice - extraordinary moves to prevent Trump's congressional allies from revealing the identity, according to three officials familiar with the discussions.

Democratic investigators are concerned that without such rare precautions, Republicans on the House Intelligence Committee could learn and then leak the identity of the whistleblower, who has agreed to answer questions before the intelligence committees in both the House and Senate.

Also on Monday, a State Department official declined to appear at a planned deposition by House committees seeking to learn more about Trump's pressure on Ukraine. The official, George Kent, is the deputy assistant secretary of state in the European and Eurasian Bureau.

"Democrats originally scheduled him for deposition today, but his appearance has not yet been worked out or confirmed," said a person familiar with the planned deposition who spoke on the condition of anonymity because they were not authorized to speak publicly.

Last week, Secretary of State Mike Pompeo balked at the timing of several planned depositions of State Department officials, saying House Democrats were not giving them adequate time to prepare.

House investigators are planning to hear Tuesday from Gordon Sondland, the U.S. ambassador to the European Union who has become a central figure in the probe, and Friday from Marie Yovanovitch, the former U.S. ambassador to Ukraine who was recalled from the post early.

Kurt Volker, the Trump administration's former special envoy for Ukraine, announced Monday that he is resigning as executive director of the McCain Institute. Volker testified behind closed doors last week before three House committees and shared text messages that have become key to the impeachment inquiry.

Volker tendered his resignation as special envoy for Ukraine, a job he had held on a part-time basis for two years, on Sept. 27 as he became a focus of the controversy over Trump's communications with Ukraine's president.

Volker said he is parting ways with the McCain Institute, which he has led since its creation in 2012, because the media's focus on his work as special envoy "risks becoming a distraction from the accomplishments and continued growth of the Institute."

The institute, which is part of Arizona State University, bills itself as a think tank "inspired by the leadership" of the late senator John McCain, R-Ariz.

"I know the Institute is well equipped with a first rate team of staff and Trustees to continue its progress in the future," Volker said in a statement.

As the impeachment inquiry pressed forward, Republicans stepped up their attacks on House Speaker Nancy Pelosi, D-Calif. President Donald Trump suggested in late-night tweets that she should be removed from office.

In a Monday tweet, Trump's personal lawyer, Rudy Giuliani, resurrecting his idea of filing a lawsuit against Pelosi for "conspiracy to violate constitutional and civil rights."

Pelosi sought to parlay the attacks into donations for the Democratic Congressional Campaign Committee.

"Late last night, President Trump took to Twitter to spew personal attacks at me," Pelosi said in the solicitation. "Well, I have news for him: All he's done is make me more determined than ever to keep fighting to reveal the truth about his abuses of power."

Meanwhile, Vice President Mike Pence suggested Monday that Democrats pursuing the impeachment inquiry have their priorities out of order.

"While Dems in Congress have been trying to overturn the will of the American people by reversing Election Day 2016, our Admin will continue to fight for policies that create jobs & benefit American workers," Pence wrote in a tweet, quoting from an op-ed he wrote for the Arizona Daily Star.

This article was written by Brittany Shammas and John Wagner, reporters for The Washington Post. The Washington Post's John Hudson, Deanna Paul and Tim Elfrink contributed to this report.