Duke says no water was flowing through the dam.
Steve McEvoy of the state Division of Energy, Mineral and Land Resources says inspectors are trying to determine the cause of the crack, and whether the opening is a threat to the integrity of the structure.
But he says the dam doesn't appear to be in imminent danger of failing.
"This is the latest in a series of troubling incidents at Duke Energy facilities over the past few months, and it's time for Duke Energy to come out of the shadows and to publicly address this growing problem," said Gov. Pat McCrory. "Initial reports show that the dam does not appear to be in imminent danger of failure. We are going to continue to enforce the law and take appropriate action to address this situation. We need an explanation from Duke Energy as soon as possible ? not only to us, but to the people of North Carolina."
The structure is at the Cape Fear River plant, where North Carolina regulators said Thursday that Duke illegally pumped 61 million gallons of contaminated water from a coal ash pit.
Regulators say the dumping violated Duke's wastewater permit at its Cape Fear plant.