Treasury Secretary Steven Mnuchin’s top aide flew on a hedge fund billionaire’s private jet to Palm Beach, Fla., several months ago, people familiar with the trip said, the latest example of senior Trump administration officials using luxury air travel even though it often raises red flags with ethics officials.

Eli Miller, Mnuchin’s chief of staff, flew with Nelson Peltz, a founding partner of New York-based Trian Fund Management, on the trip. Peltz is an activist shareholder who has sought a board seat at Procter & Gamble, seeking to shake up management. He has spoken glowingly about Trump's proposal to slash tax rates on businesses and the wealthy, which is something designed in large part by senior Treasury officials.

Rich Delmar, counsel to the Treasury Department’s inspector general, said the office has launched an inquiry into Miller’s trip on the private jet following a Washington Post report.

A Treasury Department spokesman confirmed the trip but denied there were any ethical issues.

“The Treasury ethics office advised Mr. Miller that he was permitted to accept a seat on a plane from a friend with whom he has a preexisting relationship under federal ethics law,” the spokesman said.

Anne Tarbell, a spokeswoman for Trian Fund Management, said, “We have no comment.”

The precise date of the trip could not be learned.

The federal government has rules aimed at preventing individuals from trying to affect government policy by giving gifts to public officials. There are exceptions that often depend on whether a gift was given in a personal capacity or as a business expense. Travel costs can raise the most scrutiny.

“There has to be a legitimate relationship” in order for a trip like this to be allowed by ethics officials, said Walter Shaub, who recently stepped down as director of the U.S. Office of Government Ethics. “You are looking to see if this is being done because of the job or because of a friendship.”

A number of senior Trump administration officials are facing scrutiny because of their use of government and charter planes for travel, sparking several inspector general investigations. On Friday, Health and Human Services Secretary Tom Price resigned after revelations that he had taken private jets at taxpayer expense on a number of trips. Also on Friday, the White House gave Chief of Staff John F. Kelly new authority to sign off on government travel for government-owned, rented, leased or chartered aircraft.

Peltz is a major political contributor, giving more than $220,000 in donations for the 2016 election cycle, according to the Center for Responsive Politics. The majority of it went to Republicans, the center said.

Miller, who is in his mid-30s, was a key adviser to Trump during the 2016 election, serving as the campaign's chief operating officer. In that role, he worked closely with Mnuchin, who was the campaign’s national finance chairman. Miller worked for former U.S. representative John A. Boehner (R-Ohio), was a senior aide to 2012 Republican presidential nominee Mitt Romney and served as Ohio state director of the Koch-funded conservative advocacy group Americans for Prosperity.

Hedge funds often have interest in the Treasury Department’s operations, as Treasury has a unique influence over the White House’s decisions related to financial markets, debt decisions and new policy proposals.

Mnuchin’s air travel is already under scrutiny by the Treasury Department inspector general. Mnuchin and his wife flew on a government plane to Kentucky in August, a trip that officials would normally take on a commercial airline.

Mnuchin also inquired about the possibility of flying on a government plane to Scotland over the summer for his honeymoon, though he never followed through with that effort.

Peltz has a large estate in Palm Beach that several media outlets have said is worth between $100 million and $200 million.