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.

“The Defense Department has not produced a single document in this investigation,” said a senior Democratic aide on the oversight panel. “The committee will be forced to consider alternative steps if the Pentagon does not begin complying voluntarily in the coming days.”

The Pentagon, Air Force and White House did not immediately respond to a request for comment.

On previous trips to the Middle East, the C-17 had landed at U.S. air bases such as Ramstein Air Base in Germany or Naval Station Rota in Spain to refuel, according to one person familiar with the trips. Occasionally the plane stopped in the Azores and once in Sigonella, Italy, both of which have U.S. military sites, the person added.

But on this particular trip, the plane landed in Glasgow — a pitstop the five-man crew had never experienced in their dozens of trips to the Middle East. The location lacked a U.S. base and was dozens of miles away from the crew’s overnight lodging at the Turnberry resort.

Had the crew needed to make a stop in the U.K., Lakenheath Air Base is situated nearby in England. The layover might have been cheaper, too: the military gets billed at a higher rate for fuel at commercial airports.

One crew member was so struck by the choice of hotel — markedly different than the Marriotts and Hiltons the 176th maintenance squadron is used to — that he texted someone close to him and told him about the stay, sending a photo and noting that the crew’s per diem allowance wasn’t enough to cover food and drinks at the ritzy resort.

The revelation that an Air Force mission may have helped line the president’s pockets comes days after Vice President Mike Pence was pressed about his decision to stay at Trump’s property in Doonbeg, Ireland, despite its location hundreds of miles away from his meetings in Dublin. The Oversight Committee is also investigating Pence’s stay at the resort.

Accusations that Trump’s properties are unfairly profiting off of his administration have dogged the president since entering office. Ethics officials and lawmakers have raised concerns about foreign officials staying at Trump hotels, and noted that Trump supporters and industry groups regularly throw bashes at Trump-owned locations. Trump is also considering hosting next year’s Group of Seven gathering of world leaders at his Doral resort in Florida, a potential financial boon for the property, and has previously stayed at the Turnberry property.

But the potential involvement of the military takes the issue to a different level.

A senior Air Force official who was previously stationed at Elmendorf Air Force Base in Alaska — where the C-17 crew was based — said choosing to refuel in Glasgow and stay at a posh property a half hour away would be unusual for such a mission. Typically, the official said, air crews stay on a military base while in transit or at nearby lodgings “unless all the hotels are booked or there is a Scottish sheep festival going on.”

The official, who was not aware of the specific allegations, also said that the mid-level officers or senior enlisted airmen commonly responsible for identifying lodging for the personnel are notoriously frugal and try to stay where their government allowance covers the costs.

“Master sergeants are cheap,” he said.

Several weeks after being alerted to the curious overnight stop, the Oversight Committee wrote a letter to acting Defense Secretary Patrick Shanahan asking for documents related to Defense Department expenditures at Trump Turnberry and the nearby Glasgow Prestwick Airport.

The letter, signed by signed by House Oversight Chairman Elijah Cummings and Rep. Jamie Raskin (D-Md.), notes that U.S. military expenditures at the airport “appear to have increased substantially since the election.”

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.

The letter asks the Pentagon for all communications between the Defense and State Departments related to “per diem” allowances in Scotland, as well as “all pre-audit flags related to air crew travel” through the Prestwick Airport and Turnberry resort, “or travel allowances beyond the normal allocations in the Defense Travel System.”

Connor O’Brien contributed to this report.