Federal authorities have launched a criminal investigation into a massive coal ash spill into a North Carolina river, demanding that Duke Energy and state regulators hand over reams of documents related to the accident that left a waterway polluted with tons of toxic sludge.

The US attorney’s office in Raleigh issued grand jury subpoenas seeking records from Duke and the North Carolina Department of Environment and Natural Resources (DENR). The subpoenas seek emails, memos and reports related to the 2 February spill into the Dan River and the state’s oversight of the company’s 30 other coal ash dumps.

On Thursday, the Associated Press obtained a copy of the subpoena issued to the state.

“An official criminal investigation of a suspected felony is being conducted by an agency of the United States and a federal grand jury,” said the subpoena, dated Monday.

The exact crime and precisely who is being targeted for potential prosecution is not spelled out in the document.

A Duke spokesman confirmed the nation’s largest electricity provider had also received a subpoena.

Thomas Walker, the US attorney for the eastern district of North Carolina, said he could not comment on the subpoenas.

The spill at a Duke Energy plant in Eden spewed enough toxic sludge to fill 73 Olympic-sized pools, turning river water a milky gray for miles. It was the third-largest coal ash spill in US history. State health officials have advised that people not eat fish from the river and to avoid contact with the water.

Prosecutors ordered the state environmental agency’s chief lawyer to testify next month before a grand jury. An agency spokesman, Drew Elliot, said the state will fully cooperate with the federal investigation.

Duke Energy spokesman Thomas Williams said the company doesn’t comment on pending litigation but said officials would cooperate with any investigation.

The subpoenas were issued the day after the AP reported that environmental groups have tried three times in the past year to sue under the Clean Water Act to force Duke to clear out leaky coal ash dumps. The groups sued after North Carolina regulators failed to act on evidence that conservationists said showed groundwater contamination.

Each time, the state agency blocked the citizen lawsuits by intervening at the last minute to assert its own authority under the act to take enforcement action in state court. After negotiating with the company, the state proposed settlements that environmentalists regarded as highly favourable to the company.

Duke, a company valued at $50bn, would have paid fines of $99,111 for groundwater pollution leaching from two coal dumps like the one that ruptured last week. The settlement would have required Duke to study how to stop the contamination, but it included no requirement to clean up the dumps near Asheville and Charlotte.

Among the documents targeted by the federal subpoenas are the correspondence between Duke and the state environmental agency related to the proposed deal highlighted in the AP’s story. On Monday, lawyers for the state asked a judge to disregard the agency’s own proposed settlement in the wake of the spill.

The criminal investigation is a serious new development, said Frank Holleman, senior attorney for the Southern Environmental Law Center, the group that had tried to sue Duke under the Clean Water Act.

After the SELC’s third attempt, the state also filed enforcement actions for all of Duke’s remaining coal ash sites in North Carolina, which effectively blocks environmentalists from pursuing action against them under the Clean Water Act.

“The state stated under oath in August that Duke was violating the Clean Water laws because of its unpermitted discharges of pollution from the coal ash lagoons into the Dan River. That was six months before this spill,” Holleman said.

Yet during that time, the state did nothing to stop Duke from polluting the river, he said.

He said he hopes investigators press regulators on why they didn’t take action.

“If anything proves that this is a very serious situation, this subpoena and this grand jury do it,” he said. “To date, if this doesn’t get Duke’s attention and if this doesn’t get DENR’s attention, what will?”