On Thursday, House Democrats summoned Treasury Secretary Steve Mnuchin to give a classified briefing on President Donald Trump‘s administration’s recent removal of economic sanctions on firms tied to billionaire Russian oligarch Oleg Deripaska

The move marks Democrats’ latest effort to hold greater oversight over the Trump administration since they regained the majority in Congress’s lower chamber.

The administration announced the lifting of sanctions on Deripaska’s companies last month. One of these corporations, Rusal, is the second largest aluminum firm in the world. Deripaska reportedly mounted a multimillion-dollar lobbying campaign to get the sanctions removed. The Trump administration appeared to have hoped lifting the limits on the Russian’s corporate empire in the final days of the year would make it less likely for Congress to block this action.

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Deripaska is known for being closely linked to Russian President Vladimir PutinThis is relevant because Trump and many of his associates are still being probed by special counsel Robert Mueller for potential collusion during the 2016 election. The oligarch also had business affairs with Trump’s former 2016 campaign chairman Paul Manafortwho this week was revealed to have shared polling data during the election with a Russian intelligence operative named Konstantin Kilimnik

In a letter sent to Mnuchin on Tuesday, Democratic lawmakers inquired why Deripaska still held “significant ownership” of En+ — Rusal’s holding company — and why the oligarch helped transfer shares to VTB, a Russian bank that was under sanctions. The government shutdown — which entered its 20th day on Thursday — is also rendering Congress’s review of these sanctions more difficult.

Last week, New York Democratic Sen. Chuck Schumer — the Senate minority leader — pushed to block the removal of the sanctions by filing a resolution to stop the Treasury Department’s decision. However, the Senate remains under Republican control, and it appears unlikely that GOP lawmakers will oppose Trump by keeping the sanctions in place.

Last summer, Mnuchin said Rusal was a major player in the aluminum industry, which also explains why the Trump administration would be unwilling to sanction the company.