Environmental group Water Keepers released satellite images it captured during one of the inspections that showed the crack clearly marked by metal stakes topped with orange streamers.
The dam also appeared to be drooping and swollen.
The agency said it was unaware of the crack until Duke reported it on March 20, after the company was named for illegally dumping 61 million gallons of contaminated wastewater into the river.
In light of the recent ash spill the DENR may consider seeking a change to North Carolina law.
The law only requires dam owners to inform regulators when it is considered likely that the dam may fail and emergency action is proposed.
After Duke reported the crack on March 20, the state ordered the company to come up with an emergency action plan.
Duke said the crack had since been repaired and the dam was not in danger of collapse.
Despite inspectors expressing concern that the water level in the pond was low, the agency did not take any enforcement action on the issue until after the photos showing the pumping were reported in the media.