Seven Democratic senators have asked 81-year-old billionaire investor Carl Icahn to explain his advisory role within the Trump administration, arguing that his holdings, including in the energy sector, "are vast and riddled with conflicts of interest" and raise "alarming questions."

Last December, Icahn, who as an investor wielded influence over several U.S. energy companies, including Chesapeake Energy Corp. and Freeport-McMoRan Inc., was tapped by President Trump to serve as a special advisor on issues relating to regulatory reform.

But in a letter to Icahn on Monday, Sen. Sheldon Whitehouse (D-RI) and six of his counterparts said they "are increasingly concerned about the role you are playing in the Trump administration and the possibility that you are breaking federal conflict of interest laws.

"Reports suggest you are unfairly using your position in the White House and your access to President Trump to influence the nation's regulatory policies in order to serve your own interests and not those of the American people."

According to Whitehouse, several lawmakers have written to White House Counsel Don McGahn, seeking clarification over Icahn's role and whether the billionaire was complying with financial disclosure requirements. He said none of lawmakers received a response.

Case in point, Whitehouse took note of Icahn's 82% stake in CVR Energy Inc., a Texas-based oil refiner that owns unconventional oil and gas-related infrastructure developments across the U.S. onshore and a market capitalization valued at $1.6 billion. Whitehouse said Icahn had an active role in the selection of Scott Pruitt to serve as administrator of the U.S. Environmental Protection Agency (EPA), and that he "personally pressed Pruitt to support changes" to the EPA's Renewable Fuel Standard (RFS).

Although Whitehouse said RFS changes "would save CVR Energy hundreds of millions of dollars annually," Reuters reported that the White House denied that an executive order on changing RFS was in the works.

Whitehouse added that despite Icahn and his representatives downplaying his role within the Trump administration, filings by Icahn Enterprises LP to the U.S. Securities and Exchange Commission (SEC) "cast your relationship in a different light." He argued the conglomerate told the SEC about Icahn's role because it presumed its shareholders would consider the news important.

"You cannot have it both ways," Whitehouse wrote. "If your involvement with the administration is not significant enough to require you to play by the conflict of interest rules required of other advisors to the president, [then] it cannot also be significant enough to require you to advise your company's shareholders of the materiality of your White House position."

Whitehouse said that if media reports on Icahn's role within the administration are accurate, the billionaire should be classified as a special government employee. But if that's the case, Whitehouse said Icahn appears to be violating the rules applicable to that type of post. He then listed 15 questions for the billionaire to answer -- including whether he had been required to divest of any holdings or recuse himself from any applicable deliberations, or if he advised the president, EPA or White House officials over RFS policy.

"The longer you and the administration refuse to answer questions about your role in the administration, and refuse to take all appropriate steps required under law to address your conflicts of interest, the greater the appearance that you are merely using your access for personal gain," Whitehouse said. "This is absolutely inconsistent with the president's claim that he is 'drain[ing] the swamp.'"

The letter was also signed by Sens. Tammy Baldwin (D-WI), Sherrod Brown (D-OH), Tammy Duckworth (D-IL), Patrick Leahy (D-VT), Jeff Merkley (D-OR), Elizabeth Warren (D-MA). Copies were also sent to Attorney General Jeff Sessions; Michael Piwowar, acting chairman of the SEC; and Walter Shaub, director of the Office of Government Ethics.

According to Forbes, Icahn's net worth is estimated at $16.5 billion. In the energy sector, he has a reputation of using his wealth to compel companies to bend to his wishes.

In the wake of struggling finances and mismanagement, Icahn became Chesapeake's second-largest shareholder in 2010. He was considered instrumental in forcing out board members and eventually, CEO Aubrey McClendon in 2013.

In 2015, Icahn acquired an 8.46% stake in offshore operator Freeport-McMoRan, which has since undergone a major overhaul. Late last year, Icahn was allegedlyinstrumentalin ousting Cheniere Energy Inc. CEO and co-founder Charif Souki.

Icahn and his affiliates have also reportedly been involved with management changes at Cheniere Energy Inc., Anadarko Petroleum Corp., Dynegy Inc. and Talisman Energy Inc., among others.