In early Spring of this year, an Air National Guard crew made a routine trip from the U.S. to Kuwait to deliver supplies.

What wasn’t routine was where the crew stopped along the way: President Donald Trump’s Turnberry resort, about 50 miles outside Glasgow, Scotland.

Since April, the House Oversight Committee has been investigating why the crew on the C-17 military transport plane made the unusual stay — both en route to the Middle East and on the way back — at the luxury waterside resort, according to several people familiar with the incident. But they have yet to receive any answers from the Pentagon.

The inquiry is part of a broader, previously unreported probe into U.S. military expenditures at and around the Trump property in Scotland. According to a letter the panel sent to the Pentagon in June, the military has spent $11 million on fuel at the Prestwick Airport — the closest airport to Trump Turnberry — since October 2017, fuel that would be cheaper if purchased at a U.S. military base. The letter also cites a Guardian report that the airport provided cut-rate rooms and free rounds of golf at Turnberry for U.S. military members.

Taken together, the incidents raise the possibility that the military has helped keep Trump’s Turnberry resort afloat — the property lost $4.5 million in 2017, but revenue went up $3 million in 2018.

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Prestwick Airport has long been debt-ridden. The Scottish government bought it in 2013 for £1, but it has continued to lose money in the years since. In June, the government announced its intent to sell the airport, which the panel’s letter described as “integral” to the success of the Turnberry property, 30 miles away.

Because of that, the lawmakers argued that the spending at the airport — in addition to the spending at the Trump property — raises concerns about conflicts of interest and possible violations of the domestic emoluments clause of the Constitution, which prohibits the president from receiving any compensation from the federal government other than his salary. After being elected, Trump chose not to fully divest himself from his business interests, choosing instead to put his holdings in a trust that he can receive money from at any time.