Duke Energy, the electric utility whose massive spill of toxic coal ash into a river six weeks ago is part of a federal investigation, illegally pumped as much as 61 million gallons of coal-ash wastewater into a second river from September to last week, North Carolina regulators charged on Thursday.

Both the accidental spill and the deliberate releases occurred not far upstream from municipal drinking-water intakes.

The utility’s officials have said that the pumping was part of preparations for routine maintenance of two settling ponds that hold ash, the remains of coal burned to generate power.

But regulators cast doubt on that claim on Thursday. “The state’s investigation revealed that the pumping activities ongoing at this plant far exceeded what would reasonably be considered routine maintenance,” said Tom Reeder, the director of the water resources division at the state Department of Environment and Natural Resources.