Four 2016 presidential campaign committees owe a combined $3.9 million to the Secret Service after the agency overpaid the campaigns in reimbursements for travel costs for agents who accompanied candidates and their families.

The Government Accountability Office issued a report Thursday analyzing the debts owed by the campaign committees of President Donald Trump, Housing and Urban Development Secretary Ben Carson, Hillary Clinton, and Vermont Sen. Bernie Sanders.

GAO investigators found the Secret Service committed numerous errors documenting lodging bills and calculating travel reimbursement amounts. When the agency discovered it had been misinterpreting internal policies for reimbursing campaigns, it did not ensure that agents knew to change how they paid back campaigns for travel costs.

“The Secret Service is incredibly proud of how we operated during the 2016 campaign, and like any high performance operation we are committed to continual improvement,” the agency said in a statement Thursday. “After the issues highlighted in the report were brought to our attention, the Secret Service took immediate action to address them.”

The agency has not yet recovered money from the campaigns for the overpayments.

Federal agencies are usually required to collect debts owed to the U.S. government that have been determined by an appropriate official, according to the GAO report. Those debts include overpayments.

So the Secret Service must now collect the $3.9 million in overpayments.

The problem?

Some candidates, especially Clinton and Carson, have mostly emptied their campaign committee coffers. Clinton’s Hillary for America committee has just over $1 million left, per the latest Federal Elections Commission report. Carson’s Carson America committee has only $461,000 remaining in cash.

It is up to the Secret Service to determine how it should proceed to collect on the identified debts, the GAO report states.

The Secret Service spent roughly $58 million on its agents’ travel expenses during the 2016 presidential campaign. Of that $58 million, $17.1 million was spent reimbursing the Trump, Clinton, Sanders, and Carson campaign committees for chartered aircraft flights.

But they overshot the amount they owed the committees for those flights by $3.9 million — almost a 30 percent overage.

Since 1977, the report said, the Secret Service has paid the lower of two airfare calculations when reimbursing campaign committees for the cost of booking agents tickets: the lowest-priced commercially available first-class ticket to the destination; or the pro rata fare of the cost of a charter — that is, the total cost of the chartered flight divided by the total number of people aboard the plane.

The GAO initiated its investigation at the request of Homeland Security ranking member Bennie Thompson of Mississippi.

It was “unacceptable” that the Secret Service overpaid for its travel expenses by nearly $4 million, Thompson said Thursday.

“Worse yet, they did not fix the error when it was discovered months before the election,” Thompson said. “Proper stewardship of Secret Service travel expenses is critical as they are paid by the American taxpayer — not the campaigns.”

Thompson called on the agency to quickly collect the debt for its overpayments.