In a letter this week, Sen. Ron Johnson (R-Wisconsin) threatened to issue a subpoena for documents related to and witnesses familiar with former Vice President Joe Biden‘s son Hunter‘s service and role as chairman on the board of the Ukrainian Gas company, Burisma Gas Holdings.

Johnson sits chairman on the Senate Homeland Security and Governmental Affairs Committee.

The committee is one of many in the Republican-led Senate that is launching a further inquiry into Hunter’s service on the board. At the same time, his father sat as vice president of the U.S. and led international anti-corruption efforts in Ukraine.

Johnson stated that he would soon schedule a committee conference meeting to hold a vote on obtaining documents from Blue Star Strategies – a Democratic-based consulting and public affairs firm. He also stated that he would like to subpoena witness Andrii Telizhenko, a former Ukrainian diplomatic aide who was employed by Blue Star Strategies.

In Johnson’s letter to the committee members, he cited government documents indicating that the firm “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.”

During the impeachment proceedings, President Donald Trump had tried to shift the focus to Hunter’s work for Burisma.