(CNN) Treasury Secretary Steven Mnuchin is facing intensifying pressure by congressional Democrats to provide more details over the Trump administration's decision to remove sanctions on three Russian firms tied to billionaire oligarch Oleg Deripaska.

Lawmakers from both chambers sent four separate letters to Mnuchin on Tuesday requesting further details on the Treasury Department's decision to rescind sanctions on Rusal, the world's second-largest aluminum producer, as well as EN+ Group and JSC EuroSibEnergo. The letters asked for details on possible conflicts of interest.

Rep. Elijah Cummings of Maryland, the chairman of the House Oversight and Reform Committee, and Sen. Ron Wyden of Oregon, the top Democrat on the Senate Finance Committee, wrote to Mnuchin citing press reports asking how he managed his own potential personal and professional conflicts of interest with a major Rusal shareholder, Len Blavatnik.

A second letter from California Democrat Rep. Jackie Speier also called into question how their relationship may have influenced the deal.

In a tweet responding to the Speier letter, Tony Sayegh, assistant secretary for public affairs at the US Treasury, said her "concerns are premised on false information. As our response to her letter will make clear, @stevenmnuchin1 had no direct business relationship with Mr. Blavatnik and any suggestion of a conflict of interest is baseless."

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