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(CNN) —  

House Democrats on Monday expanded their impeachment inquiry with subpoenas to the Pentagon and Office of Management and Budget tied to the freezing of foreign aid to Ukraine.

The newest subpoenas broaden the House’s impeachment investigation into President Donald Trump’s interactions with Ukraine into new corners of the federal government. They follow subpoenas that have already been issued to the State Department and White House, as well as the President’s personal attorney Rudy Giuliani, as Democrats seek to rapidly gather information as they consider whether to impeach the President.

The committees have also requested information from Vice President Mike Pence as part of the probe.

The subpoenas to the Pentagon and OMB are related to the decision to hold up foreign aid to Ukraine, which came at the same time that Giuliani and Trump were pushing for Ukraine to launch an investigation into the 2016 US election, then-Vice President Joe Biden and his son, Hunter Biden. There is no evidence of wrongdoing in relation to Ukraine by the Bidens.

LIVE UPDATES: The latest on the Trump impeachment inquiry

“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 chairmen wrote.

It’s not clear how the agencies will respond to the subpoenas. The State Department missed a Friday deadline to turn over documents that Democrats’ subpoenaed, and the White House has indicated that it will not comply with the subpoena issued on Friday, arguing that the House needs to first take a full vote on authorizing an impeachment inquiry.

Last week, however, the Pentagon’s chief legal officer requested that Defense Department agencies identify, preserve and collect any and all documents relating to the provision of security assistance to Kiev.

Giuliani associates could get subpoenas

More subpoenas could be on the way, too.

Democrats have requested information from three Giuliani associates — Lev Parnas, a Ukrainian-American businessman who worked with Giuliani; Igor Fruman, a business partner of Parnas; and Semyon “Sam” Kislin, a former aide to Giuliani — and they’re warning they will subpoena three Giuliani associates if they do not comply with their requests for documents and depositions.

They have not cooperated so far, a source said. Two were supposed to testify at the end of this week and another next week. And the deadline to turn over documents is Monday.

“While we have engaged with counsels for these witnesses, they have so far refused to agree to testify or turn over relevant documents - if they continue to fail to comply, they will be served with subpoenas in short order,” an official working on the impeachment inquiry said.

John Dowd, who is representing Parnas and Fruman, wrote in a letter to the committees that some of the information they’re seeking is protected by attorney-client privilege, and a review of the material cannot be conducted in just the week given to comply with the request. He said he expected to meet with them this week and review the documents requested, but said that responding to the committees “will take some time.”

“Your request for documents and communications is overly broad and unduly burdensome. The subject matter of your requests is well beyond the scope of your inquiry,” Dowd wrote. “This, in combination with requiring immediate responses, leads me to the inescapable conclusion that the Democratic committee members’ intent is to harass, intimidate and embarrass my clients.”

Sondland testifying this week

The Democrats’ investigation stems from a whistleblower alleging the President solicited help from Ukraine to investigate a political rival in his July 25 call Ukrainian President Volodymyr Zelensky. The complaint also notes that foreign aid to Ukraine had been suspended around the same time.

The aid was subsequently released by the Trump administration in September, but Democrats are now probing the reasons is was held up in the first place.

In text messages provided by former US special envoy Kurt Volker to Congress last week, senior US diplomat in Ukraine Bill Taylor expressed alarm that aid was being frozen, suggesting it was tied to the request for an investigation into Biden.

“Are we now saying that security assistance and WH meeting are conditioned on investigations?” Taylor texted US Ambassador to the European Union Gordon Sondland on September 1.

“As I said on the phone, I think it’s crazy to withhold security assistance for help with a political campaign,” Taylor texted a week later.

Sondland responded to Taylor’s second text that he was “incorrect” about Trump’s intentions.

“Bill, I believe you are incorrect about President Trump’s intentions,” Sondland said. “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 Zelensky promised during his campaign.”

But the texts show how the Ukrainians’ efforts to secure a meeting between Trump and Zelensky was in fact linked to the opening of an investigation ahead of the July 25 phone call.

Sondland appears behind closed doors before the House Intelligence, Oversight and Foreign Affairs Committees on Tuesday.

This story has been updated with additional developments Monday.

CNN’s Manu Raju and Gloria Borger contributed to this report.