WASHINGTON — A House Democrat is demanding answers about an alleged business deal that Treasury Secretary Steven Mnuchin had with an associate of a Russian oligarch whose companies recently received US sanctions relief.

California Rep. Jackie Speier, a member of the House Intelligence Committee, sent a letter to Mnuchin last week seeking answers about a deal he reportedly made in 2017 with an associate of Oleg Deripaska’s, the billionaire aluminum magnate whose companies Treasury surprisingly announced it would be taking off the formal sanctions list in December. The letter, sent Wednesday, was obtained by BuzzFeed News.

Mnuchin’s department has been embroiled in a fight on Capitol Hill over Treasury’s decision to lift sanctions on companies controlled by Deripaska, the oligarch connected to President Donald Trump’s former campaign chair, Paul Manafort. Democrats in Congress — and some Republicans — tried unsuccessfully in recent weeks to block Treasury from lifting the sanctions, which initially shook aluminum markets and reportedly cost Deripaska billions.

Speier said she wants to learn more about Mnuchin’s involvement with Leonid Blavatnik, a Ukrainian-born billionaire with whom he co-owned a Hollywood film company up until 2017. Mnuchin had to divest from RatPac-Dune Entertainment as part of his confirmation process and disclosed the sale of his shares, but the exact prices and purchasers are unknown. However, the Hollywood Reporter reported that Mnuchin sold the shares to Blavatnik for approximately $25 million.

In the letter, Speier wrote that she had “serious concerns” about the alleged transaction, adding that it is “especially alarming” given a recent New York Times report saying that Deripaska might have gotten a better deal with Treasury than has been publicly disclosed.

Blavatnik, the letter notes, co-owns Sual Partners with another sanctioned oligarch, Viktor Vekselberg, and Sual “is a major shareholder” of Rusal, one of Deripaska’s sanctioned companies. It also notes that Blavatnik formerly served on Rusal’s board, and that one of his companies donated $1 million to Trump’s inaugural fund.

“Blavatnik had a clear financial interest in the outcome of the Treasury action,” Speier wrote.

The letter acknowledges that Mnuchin disclosed the sale of the shares, but argues that Mnuchin’s relationship with Blavatnik and involvement in easing sanctions “is clearly a conflict of interest.”

The letter asks a series of specific questions about the sale of the RatPac shares, including the date of the transactions, names of purchasers, value of the sales, whether Mnuchin sought ethics advice beforehand, and more. The letter also asks whether Mnuchin has sought ethics guidance related to his involvement in the sanctions issue more broadly. Moreover, the letter asks whether Blavatnik has tried to communicate with Mnuchin directly about the sanctions.

The Treasury Department did not respond to a request for comment.

On Dec. 19 — three days before the federal government was set to shut down — the Treasury Department announced that “significant restructuring and corporate governance changes” at three Deripaska-controlled companies, including Rusal and En+, would “enable them to meet the criteria for delisting.” Treasury first imposed the sanctions on Deripaska, a close confidant of Russian President Vladimir Putin, and his companies in April 2018 in response to Russian interference in the last presidential election.

The move received immediate skepticism from Congress, including the Senate Intelligence Committee, which is investigating Russian election interference and potential collusion between the Trump campaign and the Kremlin. Sens. Richard Burr and Mark Warner, the committee’s chair and vice chair, respectively, said Treasury’s announcement did “not change the fact that Mr. Deripaska, his employees, and his companies work at Vladimir Putin’s behest and operate as de facto representatives of the Russian government — a government that has occupied and intimidated its neighbors, sought to disrupt free and fair elections, violated nuclear treaties, and continued to wage influence campaigns to undermine western democracies, including our own.”

Lawmakers have expressed frustration at the level of information Treasury has provided on the decision to ease the sanctions, which were lifted Sunday. Mnuchin delivered a classified briefing to all House members three weeks ago about the plan, but Speaker Nancy Pelosi called it “one of the worst classified briefings” they had received from the administration.

On Tuesday, a group of top House Democrats issued a statement saying they were “deeply troubled” by Treasury’s decision to lift the sanctions. The statement, signed by the chairs of the House Ways and Means, Foreign Affairs, Intelligence, and Financial Services committees, also criticized Mnuchin for denying lawmakers’ requests for more time to evaluate Treasury’s decision to ease sanctions.

“We are considering additional legislative actions to ensure that Treasury and these companies comply with the agreement in letter and in spirit, and to prevent something like this from happening again in the future,” the statement said. “More importantly, we will continue our oversight of Treasury’s decision by examining the terms of this deal and its implementation to safeguard against harmful actors like Mr. Deripaska benefitting from the Treasury’s delisting decisions. We do not believe termination of the sanctions has relieved Treasury of its obligation to explain fully this deal to Members.”

For her part, Speier wants information on Mnuchin’s dealings. “The fact that you did not recuse yourself from deliberations surrounding the proposed termination of sanctions on RUSAL and En+ is deeply troubling and a conflict of interest,” she wrote.

The letter requests that Mnuchin provide responses within two weeks of receiving it, and argued that Treasury should have delayed lifting the sanctions until lawmakers had time to review the requested information.