The Senate Ethics Committee on Wednesday reiterated that Senators and staff may not pocket unused per diem allotments for foreign travel; the excess must be returned to the Treasury upon a trip’s completion.

Although House and Senate Rules require lawmakers and staffers to return remaining per diem money, which is used to cover lodging, meals and other on-the-ground expenses during trips overseas for official business, a Wall Street Journal investigation last year revealed that instead many lawmakers used the money to buy souvenirs or simply kept it upon their return.

“There has been recent media attention regarding the use of foreign travel per diem by Members and staff of the Senate,” the Ethics panel letter said. “We want to make sure that you and your staffs know that any unused portion of your foreign travel per diem must be returned to the United States Treasury after you return home.”

There is no way to track how per diem money is spent. Members and staffers are not required to disclose how the money was used, and the figure shows up as a lump sum per day in the Congressional Record. The program is administered by the State Department and is intended to provide travelers with local currency upon entering a country.

The amount allotted depends on the location and duration of the trip. The Wall Street Journal estimated that over two years, Members spent at least 5,300 days visiting 130 countries and received from $375,000 to $625,000 for per diem expenses. There is no way to calculate whether the unused money was repaid, though some lawmakers reported doing so.

In a letter circulated Wednesday, the Senate Ethics Committee advised offices and committees taking trips to reread the manual governing official foreign travel.

“Travelers returning an unused per diem to the U.S. Treasury may do so by personal check or money order,” Chairman Barbara Boxer (D-Calif.) and Vice Chairman Johnny Isakson (R-Ga.) wrote. “We also understand that the [State] Department’s Congressional Travel Office will arrange for the pick-up of unused per diem.”

The House Ethics Committee in December exonerated Members who had said they pocketed excess per diem, concluding that the per diem rules were unclear and there was no proof the Members had not used the per diem money appropriately.