The Air Force has launched an international investigation into how it chooses layovers near an air base in Scotland in the wake of revelations that crew members stayed at President Donald Trump’s luxury Turnberry golf resort.

“Air Force leadership directed Air Mobility Command to review all guidance pertaining to selection of airports and lodging accommodations during international travels,” the Air Force said in a statement that was first obtained by Politico.

“Initial reviews indicate that air crew transiting through Scotland adhered to all guidance and procedures,” the statement noted. But while “lodging at higher-end accommodations” might be within government rates and “allowable,” it might not be “advisable,” it added.

“Even when USAF aircrews follow all directives and guidance, we must still be considerate of perceptions of not being good stewards of taxpayer funds that might be created through the appearance of aircrew staying at such locations,” the statement said.

The lodging — and millions of dollars in refueling stops at Glasgow Prestwick Airport just 23 miles from the Trump resort — triggered an investigation by the House Oversight Committee earlier this year to determine if taxpayer funds were being used to aid both the airport and Trump’s resort.

Crew members stayed at Turnberry during at least two refueling trips between the U.S. and the Mideast, Politico reported. A representative of the Trump Organization told The New York Times that military personnel stayed a few times.

Total payments for refueling by the Department of Defense to the commercial airport owned by the Scottish government could total as much $21 million by the time the contract ends, The Scotsman reported. The Pentagon provides Prestwick’s biggest revenue stream, though the fuel could have been obtained at an American — or allied — military base. Both Prestwick and Trump’s Turnberry have struggled financially.

The Department of Defense has so far ignored requests by the House committee for documents concerning the stays — and the Prestwick contract, according to Politico.

“The airport closest to the Trump Turnberry golf course ... has been viewed as integral to the golf course’s financial success, yet it too has lost millions of dollars every year since its purchase by the Scottish government in 2013,” committee Chairman Elijah Cummings (D-Md.) said in a letter in June to acting Defense Secretary Patrick Shanahan. “Given the president’s continued financial stake in his Scotland golf courses, these reports raise questions.”

Trump’s Turnberry resort l ost $4.5 million in 2017, but revenue increased $3 million in 2018.

The Scotsman newspaper has reported that the U.S. Defense Logistics Agency has already negotiated another contract beginning in October with Prestwick extending to 2024 for 12.4 million gallons of fuel, 3 million more than the current contract, which is about to expire.

Prestwick held discussions in 2017 about “potential partnership opportunities” with Trump Turnberry, according to documents obtained by The Scotsman’s Sunday newspaper. Prestwick is currently up for sale.

The House Oversight Committee also now plans to investigate Vice President Mike Pence’s stay at Trump’s Irish golf resort during an official visit to Ireland last week, even though the resort was 180 miles away — on the other side of the island nation — from planned meetings in Dublin.