At the time, the Trump transition team said Icahn "will be advising the President in his individual capacity and will not be serving as a federal employee or a Special Government Employee and will not have any specific duties."
After the announcement, Norman Eisen, former chief ethics lawyer to President Barack Obama, told CNBC that Icahn should beware, because the situation may pose a violation of conflict of interest and other laws, some of which could carry a criminal penalty. He said Icahn's duties may go beyond informal advising, given that he has a formal title as special advisor, sweeping responsibilities and potentially influence over personnel choices.
"It appears that he may end up as a de facto special government employee. As such, he would be subject to the conflicts rules, including under 18 USC 208," Eisen told CNBC, referring to a law that restricts people with a financial interest from advising the president.
"If so, that would make him potentially criminally liable if he worked on a repeal of the ethanol mandate that would boost CVR's stock price and enrich him. Of course, that's a lot of ifs," said Eisen, now a fellow at the Brookings Institution.
In a statement to CNBC in response to this point, Jesse Lynn, general counsel at Icahn Enterprises, said: "Mr. Icahn is well aware of his obligations under the law generally and with respect to 18 U.S.C. 208 specifically. He will follow the law as he always has."
He reiterated the Trump team's message that "unlike a government employee, [Icahn] will have no official role or duties." He added that Icahn will not be in a position to set policy, but will instead offer suggestions.
Eisen said the press release announcing Icahn's appointment did not provide sufficient details about what Icahn "will and will not do, and how he will avoid tripping into" the role of special government employee.
Richard Painter, a former chief ethics lawyer for President George W. Bush and longtime critic of Trump, told CNBC in December that it would be a "huge conflict" if Icahn advised Trump on the ethanol mandate. In an email to CNBC after the hiring, Painter noted that the Trump transition team said Icahn "is not a government employee (they may be wrong on this if he acts like one) so it looks like they are saying the rules won't apply to him."
The eight senators echoed those comments on Tuesday in their letter to the three agencies.
"These actions, and the massive profit earned by Mr. Icahn, raise clear questions about whether he may have violated conflict-of-interest rules that apply to government officials. These questions are best addressed by the Office of White House Counsel, the Office of Government Ethics, and the Department of Justice," they said.