(CNN) Special counsel Robert Mueller 's investigators have questioned a Russian oligarch about hundreds of thousands of dollars in payments his company's US affiliate made to President Donald Trump's personal attorney, Michael Cohen, after the election, according to a source familiar with the matter.

Viktor Vekselberg, chairman of asset manager Renova Group, is an oligarch close to Vladimir Putin, and last month the Trump administration placed him on a list of sanctioned Russians for activities including election interference. The purpose of the payments, which predate the sanctions, and the nature of the business relationship between Vekselberg and Cohen is unclear.

The scrutiny of the payments could add to the legal troubles for Cohen, whose home and office were raided last month as part of a criminal investigation by federal prosecutors in Manhattan. In court documents, the prosecutors said at least part of their inquiry stemmed from a referral from Mueller's office.

The questions asked of Vekselberg suggest that Mueller investigators have been examining some of Cohen's business relationships as part of the investigation into Russian meddling in the 2016 presidential election. Vekselberg is one of two Russian oligarchs the FBI stopped earlier this year after their private jets landed in New York-area airports as part of Mueller's investigation.

Investigators also asked Vekselberg about donations the head of his US affiliate made to Trump's inaugural fund and campaign funds, sources said.

The attorney for Stormy Daniels -- the porn star who received $130,000 to keep quiet about an alleged affair she had with Trump a decade ago -- produced information Tuesday evening that appears to add further details to CNN's reporting. Michael Avenatti alleged that Cohen received half a million dollars from a company affiliated with Vekselberg in the months after the presidential election.

Avenatti alleged the $500,000 went into the bank account for Essential Consultants, a shell company that Cohen set up before the election that was used to pay Daniels. Avenatti added that the payments occurred from January to August 2017.

CNN has reviewed documents that appear to show these payments. CNN has not independently authenticated the documents.

Prosecutors have not accused Cohen of wrongdoing in regard to the payments or any other business dealings.

What FBI agents wanted to know

FBI agents asked Vekselberg about payments his company's American affiliate, Columbus Nova, made to Cohen, according to one source. The Russian was questioned as well about $300,000 in political donations by Andrew Intrater, Vekselberg's American cousin who is the head of Columbus Nova, sources said.

Intrater was also questioned by Mueller's investigators, two sources told CNN. Last month, Vekselberg was placed on the US government's list of sanctioned individuals, prohibiting him from traveling to the US.

The payments occurred at the same time Cohen was trying to build a consulting and legal business after the election and around the time he left the Trump Organization. At that time, he was still Trump's personal lawyer. Even last month, as Trump responded to questions about the payment to Daniels, the President deferred to Cohen and said, "Michael is my attorney."

In court filings, Cohen's attorney said that Cohen has seven clients for whom he provides "strategic advice and business consulting." The attorney did not specify whether Columbus Nova was one of those clients.

After publication, Columbus Nova provided a statement from its attorney, Richard Owens.

"Columbus Nova is a management company solely owned and controlled by Americans," the statement said. "After the inauguration, the firm hired Michael Cohen as a business consultant regarding potential sources of capital and potential investments in real estate and other ventures. Reports today that Viktor Vekselberg used Columbus Nova as a conduit for payments to Michael Cohen are false. The claim that Viktor Vekselberg was involved or provided any funding for Columbus Nova's engagement of Michael Cohen is patently untrue. Neither Viktor Vekselberg nor anyone else other than Columbus Nova's owners, were involved in the decision to hire Cohen or provided funding for his engagement."

Later Tuesday, Eric Kosta, general counsel of Columbus Nova, sent this statement: "As the General Counsel of Columbus Nova, I can confirm that the company is 100% owned and controlled by Americans. Any suggestion that at any point in time Viktor Vekselberg or any of his companies owned or exercised any control over Columbus Nova is patently untrue."

Intrater did not respond to requests for comment. An attorney for Vekselberg did not respond to a request for comment, nor did representatives for his company in Russia. Cohen did not respond to a request for comment.

Prosecutors with the US attorney's office in Manhattan are investigating Cohen's personal financial dealings, including his taxi business and payments made to silence Daniels, sources have told CNN. The FBI raided Cohen's home, office and hotel room last month, seizing at least eight boxes of documents and more than a dozen electronic devices. Cohen's attorney said the inquiry was referred by Mueller's team.

Campaign donations under scrutiny

Vekselberg's cousin Intrater gave generously to support Trump.

He donated $250,000 to the Trump inauguration fund, $35,000 to the Trump Victory Fund, and $29,600 to the Republican National Committee in June 2017, according to the Center for Responsive Politics.

While it is illegal for foreigners to donate to US politics, Intrater is American.

However, the donations were a sharp increase from previous cycles and may raise questions for investigators. Intrater's only previous political donations included $1,200 to Democrat Bill Richardson's presidential run in 2008 and $2,600 in 2014 for Republican Chris Day's congressional race in New York. Renova Group donated between $50,000 and $100,000 to the Clinton Foundation, but it's not clear when the contribution occurred, according to the foundation's public list of donors.

Vekselberg may also be of interest to investigators because of his close ties to the Kremlin. He built his fortune following the collapse of the Soviet Union through a series of deals in the oil and gas sector. In 2004, he paid over $100 million to buy nine Faberge eggs from the American Forbes family, returning the second-largest collection of imperial eggs to Russia. Six years later, he was appointed by then-Russian president Dmitry Medvedev to lead the Skolkovo Innovation Center project, the Kremlin's answer to Silicon Valley.

From 2007 until March 2012, Vekselberg was a shareholder and chairman of the board of Rusal, the aluminum company controlled by Oleg Deripaska. Deripaska has business ties to former Trump campaign chairman Paul Manafort, who has been indicted on fraud and tax related charges as part of Mueller's investigation. Manafort has pleaded not guilty. Deripaska, who also was added to the US sanctions list last month, has sued Manafort over a soured investment deal.

Lamesa Investments Ltd, an affiliate of Renova, acquired a large stake in the Bank of Cyprus at the same time Wilbur Ross, then a private equity investor, made a capital infusion into the then-struggling bank. Lamesa now holds a 9.2% stake in the Bank of Cyprus. Ross resigned from his position as vice chairman of the bank after he was confirmed as Trump's commerce secretary.

Vekselberg also attended the December 2015 dinner in Moscow for RT, the state-controlled television channel, where Michael Flynn, Trump's former national security adviser, sat beside Putin, according to NBC News. Flynn has pleaded guilty to lying about a call he had with the Russian ambassador during the presidential transition and is cooperating with the Mueller investigation.

Update: This story has been updated to include Columbus Nova's latest statement.