The House Intelligence Committee subpoenaed two more administration officials Friday as part of its expanding impeachment investigation into President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE’s dealings with Ukraine.

Investigators subpoenaed John Eisenberg, the White House National Security Counsel’s top legal adviser, to testify on Monday. The House also subpoenaed Brian McCormack, outgoing Energy Secretary Rick Perry Rick PerryEnergy secretary questions consensus that humans cause climate change OVERNIGHT ENERGY: Democrats push resolution to battle climate change, sluggish economy and racial injustice | Senators reach compromise on greenhouse gas amendment stalling energy bill | Trump courts Florida voters with offshore drilling moratorium OVERNIGHT ENERGY: Trump signs major conservation bill into law | Senate votes to confirm Energy's No. 2 official | Trump Jr. expresses opposition to Pebble Mine project MORE’s chief of staff, for Monday deposition, according to a source familiar with the matter.

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The two subpoenas add to a growing pile of testimony House investigators plan to hear next week as the chamber seeks to ramp up its investigation.

Both Republicans and Democrats are digging in the trenches after the House’s impeachment probe resolution passed Thursday in a 232-196 party-line vote. The vote was a clear indication that an upcoming phase of the impeachment process — taking place in public hearings — will be a highly partisan affair.

The House is investigating allegations that Trump tied $400 million in aid to Ukraine to Kiev’s commitment to open an investigation into former Vice President Joe Biden Joe BidenPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Fox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio MORE, a top political rival and 2020 Democratic primary front-runner, on unfounded corruption allegations.

Several officials have already testified behind closed doors, giving both parties ammunition in the messaging war.

Tim Morrison, a senior National Security Council (NSC) aide who is leaving the White House, appeared for more than eight hours behind closed doors in the Capitol on Thursday. In a boon to Republicans, he testified that he saw nothing “illegal in Trump’s discussions with Ukraine," but he also verified previous witness accounts of a quid pro quo with Kiev.

Democrats characterized Morrison's testimony as supporting the previous and damning accounts of other witnesses such as William Taylor, the top U.S. diplomat to Ukraine, who testified last week that he believed there was a quid pro quo with regard to releasing the Ukraine aid.

Lawmakers are likely to question Eisenberg about testimony from Lt. Col. Alexander Vindman, a top Ukraine expert on the NSC, who said he raised concerns over a July call between Trump and Ukrainian President Volodymyr Zelensky in which Trump pushed his counterpart to investigate Biden.

“I did not think it was proper to demand that a foreign government investigate a U.S. citizen, and I was worried about the implications for the U.S. government’s support of Ukraine,” Vindman said in his testimony. “I realized that if Ukraine pursued an investigation into the Bidens and Burisma it would likely be interpreted as a partisan play which would undoubtedly result in Ukraine losing the bipartisan support it has thus far maintained.”

Vindman expressed his concerns to Eisenberg, who then proposed moving the call's transcript to a highly classified server, sources told The Washington Post.

Investigators are likely to press McCormack for more information on Perry’s role in dealing with Ukraine, including whether he helped convey or reinforce the alleged quid pro quo. Perry has denied any wrongdoing and refused to testify in the House’s inquiry.