AUGUSTA — Maine will likely have to repay the federal government for providing Medicaid coverage to as many as 19,000 ineligible patients during the past year and a half, Health and Human Services Commissioner Mary Mayhew said today.

“There will be an audit,” Mayhew said. And, to the extent claims were covered improperly, “there will be a requirement to pay those funds back.”

It will take the state weeks to figure out how much the state paid out improperly and how much money may have to be repaid to the federal government, Mayhew said.

Medicaid, which is called MaineCare in Maine, provides medical coverage to the poor and disabled. It is administered by the state, but about two thirds of the cost is paid by the federal government.

Mayhew said she spoke to federal officials today about the latest problem with MaineCare’s troubled computerized eligibility and claims systems. She also briefed lawmakers for a second day, answering questions about what the computer problem will cost the state and why the Legislature was not notified sooner.

Mayhew said the claims system that was launched in the summer of 2010 never communicated properly with the existing eligibility system. Between Septeber 2010 and January 2012, an estimated 19,000 MaineCare enrollees were sent letters telling them they were no longer eligible for coverage. But those same people still had active MaineCare cards and accounts and could have continued to receive coverage for medical services.

“They may not have read the letter or understood the letter, and their cards remained active,” she said.

Mayhew said the department is still working to correct the problem, and is still trying to determine how many of those 19,000 continued to get Medicaid-covered services they were not eligible for.

Besides facing a federal audit and the potential need to repay the federal government, the problem creates a new financial question mark for the department only a couple of weeks after the Legislature passed a $120 million plan to balance its budget for the current year.

Sen. Margaret Craven, D-Lewiston, was among the lawmakers asking Mayhew today why the problem didn’t come to light sooner, especially when Mayhew answered questions about the MaineCare budget from the Appropriations Committee.

“I’m asking why they weren’t told that something could be very wrong?” Craven said.

Mayhew said she didn’t know the extent of the problem until last week, when she told the governor.