WASHINGTON — Treasury Secretary Steven Mnuchin has agreed to deliver a classified briefing to U.S. House lawmakers on Thursday on his recent decision to lift sanctions on companies linked to a Russian oligarch and Vladimir Putin ally, marking the start of an aggressive new focus on Mnuchin by newly empowered House Democrats, according to two top Democratic aides.

Mnuchin, who served as the Trump campaign's national finance chairman in 2016 before being confirmed to President Donald Trump's cabinet, has largely escaped investigative scrutiny.

But because of his role in the campaign — and, most recently, the Dec. 19 announcement easing sanctions on companies aligned with Oleg Deripaska, the Putin ally with ties to former Trump campaign chairman Paul Manafort — House Democrats believe Mnuchin should be a focus of and source of information for several planned investigations both related and unrelated to the Russia probe, according to the aides. These include examinations of Trump's finances and the business practices of the Trump Organization.

The Thursday briefing comes in response to a letter sent Tuesday to Mnuchin by the new chairpersons of the seven major House investigative committees, asking him to provide answers on the Treasury Department's decision to lift the sanctions, which was announced as lawmakers left town for the holidays.

The Treasury Department said it would lift sanctions on companies linked to Deripaska — triggering a statue that gives Congress just 30 days to try to reverse the decision by passing a joint resolution of disapproval. On Friday, Senate Minority Leader Chuck Schumer, D-N.Y., filed such a motion, but it's unlikely to succeed in the Republican-run Senate.

Russian Prime Minister Vladimir Putin speaks with Oleg Deripaska at an investment forum in Sochi in 2008. Ilia Pitalev / AFP-Getty Images file

In their letter, the investigative committee heads asked Mnuchin to meet with lawmakers "in an appropriate setting." It was one of the first coordinated requests from the committees since the Democrats took control of the House last week.

Among the questions Democrats are likely to ask Mnuchin at the briefing, according to aides: How much influence did the White House, and Trump personally, exert in Mnuchin's decision to lift sanctions?

Deripaska, one of Russia's wealthiest men, is a close ally of Russian President Putin and was a business associate of former Trump Campaign Chairman Paul Manafort. Manafort has been convicted of several felony charges related to special counsel Robert Mueller's investigation and faces sentencing this spring. Deripaska is not implicated in any of those charges.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Deripaska and his companies, including Rusal, the world's second-largest aluminum company, were among dozens of Russian entities hit with Treasury Department sanctions in April as punishment for Russian interference in the 2016 U.S. election. The sanctions roiled global aluminum markets, and critics say companies in the U.S. and elsewhere were hurt.

The Treasury Department has said the decision to lift the sanctions on the companies is in exchange for Deripaska agreeing to significantly reduce his stake in them. While Deripaska will remain sanctioned, "these companies have committed to significantly diminish Deripaska's ownership and sever his control," Mnuchin said in a Dec. 19 statement. "The companies will be subject to ongoing compliance and will face severe consequences if they fail to comply," he said.

Signers of the letter, including House Ways and Means Chairman Rep. Richard Neal, D-Mass., and Financial Services Committee Chairman Rep. Maxine Waters, D-Calif., asked that the sanctions relief be postponed "until members' questions are resolved."

In fact, Democrats want answers from Mnuchin that go far beyond the sanctions decisions.

Since early 2017, when House Democrats were in the minority, Waters has asked Treasury to provide records detailing Trump's possible financial ties to Russia, as well as those of the president's family members and associates.

President Donald Trump speaks as Secretary of the Treasury Steven Mnuchin looks on during a tour of the Boeing Company on March 14, 2018 in St. Louis, Missouri. Mandel Ngan / AFP - Getty Images

Later that year, Democrats on the committee wrote to Mnuchin requesting his help in determining "the extent of any undue influence on the president and his administration from Russian government officials, oligarchs and organized crime leaders."

Among other things, Mnuchin oversees the Treasury Department's Financial Crimes Enforcement Unit, which maintains a database for law enforcement officials to combat money laundering and financial crimes. Yet he hasn't responded to inquiries from Financial Services Committee Democrats over the past two years about what they say are potentially suspicious financial dealings of the president, his family and his campaign associates, Democratic aides said.

Waters has been active in scrutinizing Deutsche Bank, which has a history of violating anti-money laundering laws and has also lent hundreds of millions of dollars to Trump and his family members. Deutsche Bank is the subject of a Department of Justice investigation related to this "mirror trading" scandal, in which more than $10 billion was secretly transferred out of Russia by a group of corrupt traders.

In a May 2017 letter to Mnuchin, Waters also raised questions about comments from Trump's son Eric, during a 2008 conference in New York, that "Russians make up a pretty disproportionate cross-section of a lot of our assets." Democratic committee chairs say they haven't gotten answers from Mnuchin on their many inquiries.

Democrats have also expressed concern that Mnuchin faces a serious conflict in his current role, given his leadership of the Trump campaign's fundraising operation, and have called for his recusal from law enforcement and regulatory issues involving the president.

As the gatekeeper for information like the Financial Crimes Enforcement Network, Democrats told Mnuchin they believe his position is untenable.

"It appears that your own involvement with the Trump campaign has resulted in an unavoidable conflict of interest which you are also attempting to ignore," top committee Democrats including Waters wrote in January 2018.

While Thursday's meeting with Mnuchin is focused on Russian sanctions relief, some Democrats are expected to press for answers about his knowledge of any financial links Trump or his businesses may have to Russia, according to a Democratic aide close to the process who spoke on the condition of anonymity.

Democratic committee chairs, as part of their overall investigative plan are expected to seek Trump's tax returns, in part to determine whether there were any financial ties or money laundering activities involving Trump and Russia during or prior to the campaign. Trump, breaking with tradition of recent past presidents, has refused to disclose his tax returns publicly.

While much of the House Democratic agenda to conduct broad oversight of the Trump administration is off to a slow start due to the government shutdown, House Democrats say they must address the lifting of sanctions on the Deripaska-linked companies now, given the 30-day deadline they face.

The Treasury Department, which would ordinarily be in a position to address NBC News’ questions, is one of the agencies affected by the partial government shutdown. The department did not respond to repeated requests for comment.