Sen. Ron Johnson, the chairman of the Senate Homeland Security Committee, is preparing to issue the panel’s first subpoena as part of an investigation into Burisma Holdings, the Ukrainian energy company linked to Hunter Biden, he said in a letter on Sunday.

Johnson, a Wisconsin Republican, told Democratic Michigan Sen. Gary Peters of his plans to subpoena Andrii Telizhenko, a former Ukrainian embassy official and former consultant for Blue Star Strategies, a firm that Burisma hired to fight against corruption allegations.

Telizhenko, who was a consultant for Blue Star from July 2016 to June 2017, has provided some documents to the committee, according to Johnson. But he says that he cannot turn over others without a subpoena because they are protected by a non-disclosure agreement.

“He cannot provide this responsible information unless he is compelled to do so by subpoena,” Johnson wrote to Peters in his letter, which was first reported by CBS News.

Republicans have been investigating whether Hunter Biden leveraged his father’s position as vice president to help Burisma, which has long been dogged by allegations of corruption.

Hunter Biden, who joined Burisma’s board in April 2014, is reportedly who linked the energy company up with Blue Star Strategies. Biden served on the board of the Truman National Security Project, a liberal national security think tank, with Sally Painter, one of Blue Star’s co-founders.

“As part of the Committee’s ongoing investigation, it has received U.S. government records indicating that Blue Star sought to leverage Hunter Biden’s role as a board member of Burisma to gain access to, and potentially influence matters at, the State Department,” Johnson said in the letter.

State Department records show that Painter and her Blue Star co-founder, Karen Tramontano, reached out to top State Department officials through 2016 to set up meetings to discuss Ukraine and Burisma. (RELATED: Emails: Burisma Consultant Linked To Hunter Biden Approached Top State Department Official At Think Tank Event)

In an email on June 27, 2016, Painter indicated that she spoke with Tony Blinken, who served as deputy secretary of state, at a Truman National Security Project event about “troubling events” in Ukraine. Painter requested a follow-up meeting with Blinken, who is now a foreign policy adviser to Joe Biden’s presidential campaign.

Tramontano, who was deputy chief of staff in the Clinton White House, also contacted State Department officials seeking a meeting regarding Burisma on Feb. 24, 2016.

One of the officials that Tramontano contacted said that she wanted to meet in order to get “a better understanding of how the U.S. came to the determination that [Burisma] is corrupt.”

Tramontano also mentioned that Hunter Biden was affiliated with Burisma, according to the email.

Johnson said that Blue Star has turned over some documents to his committee, but that the firm has said it will not be handing over any more records.

“The Committee notified Blue Star that we consider their response to be incomplete, but they responded that they will not be producing any more documents,” wrote Johnson.

It is unclear what work Telizhenko did for Blue Star or Burisma. In recent months, Telizhenko has worked closely with Rudy Giuliani as he has pushed for deeper investigations into the Bidens and Burisma.

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