Secret Service overpaid campaigns $4 million for travel during 2016 election: Report The expenses were a result of a misinterpretation of a FEC regulation.

The U.S. Government Accountability Office (GAO) released a new report Thursday that shows the U.S. Secret Service overpaid presidential campaigns by approximately $3.9 million during the 2016 election for travel.

The overpaid expenses were a result of a misinterpretation of a Federal Election Commision regulation that enables the Secret Service to “pay the lower of two applicable fares for special agents on chartered aircraft flights.”

According to the report, the error was caught eight months before the end of the presidential campaign. However, Secret Service officials deemed the calculation as “erroneous” and “did not ensure the agency reverted to its long-standing policy.”

During this time period, 66 percent of flights were taken with special agents on board until Election Day.

During the 2016 presidential election, the Secret Service was assigned to protect four presidential candidates, two vice presidential candidates, and six of the candidates’ family members. The report took into account the number of expenses incurred in travel and how accurately the agency followed the FEC regulations regarding these payments.

In total, the Secret Service spent $58 million in travel costs and reimbursed $17.1 million of that total to campaign committees for chartered aircraft flights.

The service paid President Trump's campaign $7.3 million for 965 chartered flights, former Secretary of State Hillary Clinton's campaign $7.1 million for 1,317 flights, Sen. Bernie Sanders's campaign $2 million for 159 flights and Ben Carson's campaign over $615,000 for 107 flights. The report did not detail how much each campaign owed to the agency.

As of February 2018, the Secret Service says that they will start to draft a new policy and propose changes for the charter aircraft reimbursement policy that will stay “consistent with its directive on revising policy.”

The Secret Service recently sent letters to the campaign committees requesting repayment. However, the Sanders campaign says they need more information before moving forward with any requests.

"During the campaign, we were reimbursed by the Secret Service based on amounts they calculated, consistent with federal election law,” Jeff Weaver, former campaign manager for Bernie Sanders, told ABC News in a statement. “They have contacted us by letter indicating they now think they used a formula for all the candidates that did not follow their own internal policy. However their letter does not explain the legal basis for their believing they made errors. So we have requested more information to better understand what they are claiming."