The US Secret Service overpaid presidential campaigns an estimated $4 million for plane travel during the 2016 election, a government watchdog says in a new report obtained by CNN.
Secret Service officials realized eight months before Election Day that they were using the wrong formula to calculate the costs, "but did not ensure the agency reverted to its longstanding policy," according to the Government Accountability Office report.
The Secret Service, which incurred millions of dollars in travel expenses to provide security during presidential campaigns, said it has asked the campaigns that were overpaid to provide the government refunds.
GAO said the Secret Service spent an estimated $58 million on travel expenses related to the campaigns. Of that total, $17.1 million was paid to the campaigns of Donald Trump, Hillary Clinton, Bernie Sanders and Ben Carson as reimbursements for seats occupied by Secret Service agents onboard campaign charter flights.
The report did not detail how much each campaign was overpaid, nor did it accuse the campaigns of wrongdoing.
The Secret Service did not respond to a request from CNN for more detailed figures. It said in a statement it is "working with all stakeholders in an effort to recover the overpaid amounts."
In a May 14 letter to the GAO, which is attached to the report, the Secret Service said letters were sent to campaign committees requesting repayment by May 31, 2018. The Secret Service also said it instituted a new policy that will help it make more accurate payments.
At least two of the four campaigns said they received the letter.
"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," said Jeff Weaver, the campaign manager for Sanders' 2016 bid.
Carson's business manager Armstrong Williams said he met with Secret Service officials for a "respectful" discussion. He said Carson currently has "no campaign," but declined to say whether that means the Secret Service will not be repaid.
According to Federal Election Commission records, the Carson campaign had roughly $460,000 in its account as of the end of March.
Spokespeople for the Trump and Clinton campaigns reached by CNN on Thursday did not immediately have information on the matter.
The report detailed how much the Secret Service reimbursed each campaign for travel expenses:
• Donald Trump: $7.3 million for 965 flights.
• Hillary Clinton: $7.1 million for 1,317 flights.
• Bernie Sanders: $2 million for 159 flights.
• Ben Carson: $615,567 for 107 flights.
In addition to those four candidates, the Secret Service also protected two vice presidential candidates and six of the candidates' family members during the 2016 campaign.
Rep. Bennie Thompson, the top Democrat on the House committee overseeing the Secret Service, called the overpayments "unacceptable," and urged the Secret Service to recoup the funds.
"Worse yet, they did not fix the error when it was discovered months before the election," Thompson told CNN in a statement. "The money that the agency needlessly and erroneously paid the campaigns is sorely needed to help fund critical Secret Service activities that have faced recent budget cuts -- including investigations into cyber and financial crimes."
The Secret Service's policy "has been to pay the lower of two fares when reimbursing campaign committees for special agents' travel on chartered aircraft flights," the GAO said. "The lowest commercially available first class airfare or the pro rata fare- the cost of the agent's seat on the charter flight calculated by taking the total cost of the charter flight of the charter flight divided by the number of passengers on board."
But agency policy did not require invoices and receipts from charter companies that would allow it to verify the costs were accurate, GAO found. Only Clinton's campaign provided those records.
The Secret Service said it has changed that policy and said it will now review documents to verify the government is being accurately charged.
The GAO also found the Trump campaign billed the Secret Service twice, "for three flight segments taken on March 1, 2016 resulting in a cumulative overpayment of approximately $21,000."