The spill, discovered Feb. 2, fouled the water to the extent officials urged the public to avoid contact with the water.
The Justice Department is investigating the North Carolina Department of Environment and Natural Resources, the (Charlotte N.C.) News & Observer reported Thursday.
"An official criminal investigation of a suspected felony is being conducted by the United States and a federal grand jury," Thomas Walker, the U.S. Attorney for the Eastern District of North Carolina, wrote in a letter attached to subpoenas demanding records, the newspaper said.
"The Department of Environment and Natural Resources will cooperate in this matter," DENR spokesman Drew Elliot said.
Duke Energy spokesman Tom Williams confirmed the company has received a subpoena.
"Duke Energy will continue to cooperate with any state or federal agency that might undertake an investigation of the Dan River ash release," he said.
A security guard discovered a pipe running beneath a 27-acre toxic waste pond at Duke's Dan River Steam Station had collapsed, and coal ash combined with contaminated water was draining into the river. There was no public announcement of the problem until the next day, and first reports by the DENR and Duke Energy omitted mention of the scope of the spill -- which eventually amounted to 82,000 tons of coal ash and 27 million gallons of tainted water, the Observer said.
Environmental advocates said they have tried three times in the past year to bring lawsuits against Duke to clean up defective coal ash dumps, but were pre-empted each time by the DENR. They accuse the head of the regulatory agency, John Skvarla, of engaging in a "cozy" relationship with Duke.
Skvarla, appointed to the office by Gov. Pat McCrory, a former Duke Energy executive, has said he considers the agency a "a partner" with those it regulates, the Observer reported.