Watchdog group Public Citizen asked Congress on Wednesday to investigate whether billionaire investor and unofficial Trump administration adviser Carl Icahn has engaged in illegal, unregistered lobbying in conjunction with his public bid to change an ethanol rule that would save one of his affiliated businesses $200 million annually.

Icahn raised eyebrows last week for getting the Renewable Fuels Association to reverse its position on a key proposal that would benefit him personally. The association, which lobbies for ethanol producers, agreed to a proposal to shift the responsibility for ensuring that gasoline contains a minimum volume of renewable fuels — from oil refiners to gasoline wholesalers. Icahn is the majority shareholder in CVR Energy, a refiner that cannot blend ethanol on its own, and which therefore must buy over $200 million in “renewable fuel credits” each year to follow the law. By shifting the responsibility to wholesalers, CVR would no longer have to make that purchase.

Trump tapped Icahn as his deregulatory czar in December. But as an unofficial adviser to the Trump administration, Icahn was able to maintain his prodigious financial holdings. The renewable fuels proposal struck many as an example of Icahn self-dealing — recommending changes in regulation that benefit him financially.

Now, Public Citizen is accusing Icahn and CVR of violating the Lobbying Disclosure Act of 1995. Any nongovernmental entity that crosses certain thresholds must register all lobbying activities with the government. Congress oversees compliance with this law.

Since the Trump administration insists that Icahn is a private citizen who receives no compensation as a government official, he would fall into the category of needing to register any lobbying work, according to a complaint sent to the clerk of the House and secretary of the Senate. Public Citizen argues that Icahn’s formal proposal to change the renewable fuel standard regulation, along with his reported assistance in vetting candidates for the Environmental Protection Agency, comprises lobbying activity.

“It is not lobbying to advise a candidate, but once Trump became president, Trump then became a covered official subject to the lobbying disclosure law,” energy program director Tyson Slocum wrote in the letter. “All of this has occurred with no record of any [Lobbying Disclosure Act] filings by or on behalf of Mr. Icahn, Icahn Enterprises or CVR Energy.”

In other words, either Icahn is a Trump administration official, and therefore profiting from his government service, or a private citizen, and therefore lobbying.

Failure to comply with the Lobbying Disclosure Act “may be subject to a civil fine of not more than $200,000,” according to the law’s text. And if an individual “knowingly and corruptly” fails to comply, they face a federal prison sentence of up to five years.

Icahn rebutted charges of conflict of interest in an interview with Bloomberg, saying, “I own a refinery. Who knows it better than me? Why shouldn’t I advocate?”

Update: 3:49 p.m., March 8:

Icahn responded to Public Citizen’s letter in an opinion column in The Hill. He called the letter a “gross misstatement of the facts which is similar to much of the ‘fake news’ that is unfortunately so prevalent today.” He wrote that he vetted his activities “with a number of lawyers and it is clear that no registration is required.” And he fired back that “A more worthwhile investigation would seek to uncover the dark forces behind this witch hunt.”

The corporate raider, who unions say destroyed more than 35,000 American jobs in key industries while enriching himself, cast himself as a heroic figure. “I have over the years fought the establishment by engaging with corporations overseen by lackluster boards and management, and those shareholders that followed alongside me have made close to a trillion dollars because my activities meaningfully enhanced the value of those corporations.”