(Bloomberg) – Carl Icahn's role as a special regulatoryadviser to President Donald Trump has ended afterquestions were raised by Democratic lawmakers about potentialconflicts of interest with his business dealings in oil refiningand insurance.

In a letter to Trump addressing his departure that was postedFriday on Icahn's website, the billionaire investor deniedprofiting from his advice-giving role — a possibilityraised by critics who had asked administration officials toinvestigate his work.

"Contrary to the insinuations of a handful of your Democraticcritics, I never had access to nonpublic information or profitedfrom my position, nor do I believe that my role presented conflictsof interest," Icahn wrote in the letter. "Indeed, out of anabundance of caution, the only issues I ever discussed with youwere broad matters of policy affecting the refining industry."

Icahn's departure capped a tumultuous week for the White Housefollowing controversial comments by Trump that seemed to lendlegitimacy to white supremacists and sparked a wave of CEOdepartures from presidential advisory panels. Three of the groupswere disbanded this week.

Role in decisions affecting AIG

Icahn, the majority owner of independent oil refiner CVR EnergyInc., drew criticism for pushing a change in U.S. biofuel policythat would benefit the company. Separate questions were raised inJuly regarding his role in regulatory decisions affecting AmericanInternational Group Inc., an insurer in which he he holds asignificant stake.

Icahn is worth about $18 billion, making him the 23rd richestperson in the U.S., according to the Bloomberg Billionaires Index.He primarily invests his own fortune rather than relying on moneyfrom outsiders, and has investments across industries includinghealth care, energy, metals, auto parts, casinos,telecommunications, real estate and rail cars.

Icahn said he was ending his arrangement with Trump's blessing,"because I did not want partisan bickering about my role to in anyway cloud your administration."

Pending decisions regarding AIG's status

Democratic Senators Elizabeth Warren of Massachusetts andSheldon Whitehouse of Rhode Island sent a letter in July TreasurySecretary Steven Mnuchin, asking whether there was correspondencebetween the billionaire and administration officials about AIG. Theactivist investor earlier had pushed for a break up of the insurerin order to help it escape its regulatory tag as "too-big-to-fail,"but eased his demands this year. 

"We write to seek assurances that Mr. Icahn has not providedinput on or received information" on a pending decision regardingAIG's status as a systemically important financialinstitution, the senators wrote. Icahn is one of the top fiveshareholders of AIG.

Prices for biofuel compliance credits known as renewableidentification numbers (RINs) have been volatile since Trump'selection and Icahn's appointment as special adviser.

Six Democratic senators asked the White House and EnvironmentalProtection Agency administrator Scott Pruitt to detail theirinteractions with Icahn on the issue. They argued that Icahn'sappointment ran afoul of ethics standards and presented a conflictof interest because Icahn hadn't made any obvious effort toseparate his business holdings from his broad mandate to addressregulations in his advisory role.

The value of Icahn's stake in the Texas refiner increased someby $491 million between Nov. 8 and Aug. 2, to about $1.4 billion, aperiod running from Trump's election to the day before reportssurfaced that the Trump administration was set to reject the bid torelieve refiners of the biofuels burden.

Departure cheered

Icahn's departure was hailed by some of his fiercest critics,some of whom linked the move to the EPA's decision to reject thebiofuel policy change the billionaire had advocated.

"Until a combination of public scrutiny and the lobbying ofethanol interests defeated his effort, Icahn appeared poised toleverage his special status with Trump into a regulatory tweak thatwould save his refining company $200 million annually," said RobertWeissman, president of the watchdog group Public Citizen.

Related: Icahn says AIG presentation inadequate, seeksdirector slate

"Unfortunately for Icahn, the scheme collapsed. Now that hisopportunity for personal profit seems to have vanished, so too hashis interest in regulatory policy," Weissman said.

Icahn endorsed Trump for president in September 2015, shortlyafter the real estate developer and reality television starannounced his run, and was frequently mentioned as a potentialTreasury secretary, a role he repeatedly said he had no interestin.

Named special adviser in December

Prior to being named a special adviser in December, he'd backedmany of Trump's campaign ideas. He supported tax deals to encouragecorporations to bring back overseas profits and stay based in theU.S., and backed calls to end the carried interest tax discount forhedge fund and private equity managers.

Icahn, 81, attended Trump's post-election victory party but leftin the early hours of the morning to bet about $1 billion on U.S.equities, he told Bloomberg at the time. U.S. stock prices tumbledon news of Trump's election but quickly rebounded, on the way to astring of record highs.

Related: Icahn urges drastic AIG shift, says CEO credibilityon the line

Having achieved fame and fortune as a corporate raider in the1980s, Icahn rebranded himself as an activist investor andoutspoken shareholder advocate. In recent years, Icahn has takenstakes in companies including Apple Inc., Hertz Global HoldingsInc., Xerox Corp. and eBay Inc., agitating forshareholder-enriching changes.

A message left for Icahn's spokeswoman wasn't immediatelyreturned.

Icahn, in his letter to Trump, repeatedly diminished hisinfluence over biofuel policy, describing himself as having sharedonly "limited insights" with the president. "I sincerely regretthat because of your extremely busy schedule, as well as my own, Ihave not had the opportunity to spend nearly as much time as I'dhoped on regulatory issues,"Icahn wrote.