Documents show that the U.S. military has spent more than $184,000 at President Trump's property in Scotland since August 2017, Democrats on the House Oversight and Reform Committee said in a letter to the Pentagon Wednesday.

Committee Chairman Elijah Cummings (D-Md.) and Rep. Jamie Raskin (D-Md.), an Oversight subcommittee chairman, implored Defense Secretary Mark Esper for more information on Pentagon spending at Turnberry golf course, saying taxpayer funds paid for the equivalent of more than 650 rooms.

"The data provided by the Department now indicates that U.S. taxpayer funds have been used to pay for more than three dozen separate stays involving hundreds of nights of rooms - all after the president was sworn into office," the lawmakers write.

The letter, the lawmakers' third on the topic since June, asks the Department of Defense for additional documents, adding that the committee is still unable to answer "basic questions, such as how many military personnel were involved and who authorized their stays."

"To date, the Department has produced only 21 pages of material - half of which appear to be publicly available on government websites," the lawmakers write. "The Department failed to produce any underlying invoices or travel records relating to spending at Trump Turnberry or Prestwick Airport."

The lawmakers also threaten a possible subpoena for the information should the Pentagon not begin producing the requested documents by Sept. 27.

The committee earlier this year opened an investigation into Air Force expenditures at and around the Turnberry resort after it was discovered that an Air National Guard crew stopped at the property while traveling to and from Kuwait.

The crew refueled at Prestwick Airport - the closest airport to Turnberry, outside Glasgow in southwest Scotland - which has seen an uptick in Pentagon fuel expenditures.

The military has spent more than $16 million on fuel at Prestwick from Jan. 20, 2017, through June 21, 2019, according to the Democrats' letter.

The Air Force has defended the refueling stops at Prestwick as within Pentagon guidelines, though the lawmakers point out that fuel would be cheaper if purchased at a U.S. military base.

The House investigation, which began in April, was reinvigorated after it was found that the Air Force sent crews to Turnberry up to 40 times since 2015, with some U.S. military stays stretching over multiple nights.

The figure, found in a preliminary Air Force investigation on the matter, does not indicate how many of the stays occurred since Trump was inaugurated in 2017.

The stops fall into a broader area of concern from Democrats, who have accused the president of using his office to boost his brand.

Vice President Pence earlier this month sparked a separate controversy when he stayed at Trump International Golf Links and Hotel in Doonbeg, Ireland, even though it was 182 miles away from his meetings in Dublin.

In an effort to put an end to such stays, Senate Democrats this week introduced the Heightened Oversight of Travel, Eating and Lodging Act, legislation that would ban federal officials from using taxpayer funds at properties owned by a president, vice president or Cabinet official.

Rachel Frazin contributed to this report, which was updated at 2:12 p.m.