By Colleen Jenkins

WINSTON-SALEM, North Carolina (Reuters) - U.S. prosecutors are investigating whether a crime was committed when thousands of tons of coal ash sludge spilled from a Duke Energy power plant in North Carolina in early February into a river that supplies drinking water for nearby towns.

Subpoenas were received this week by Duke Energy, which retired the coal-fired power plant in Eden in 2012, and the North Carolina Department of Environment and Natural Resources regulator, spokesmen for the two entities said on Thursday.

The U.S. Attorney's Office in Raleigh ordered officials with the state agency to appear before a grand jury next month with documents related to the spill that began on February 2, according to a copy of the subpoena provided to Reuters by the department.

The spokesman for Duke, which installed a permanent plug at the site to stop releases from the ash basin on February 8, declined to share a copy of the company's subpoena. He said the utility, the largest electric power provider in the United States, was cooperating with all state and federal investigations.

"The ash spill itself was something we thought we'd been very transparent about," Duke spokesman Tom Williams said. "We intend to fix any damage in the appropriate manner."

The company initially said as many as 82,000 tons of ash, enough to fill up to 32 Olympic-size swimming pools, spilled into the Dan River after a stormwater pipe broke under a 27-acre ash pond at the coal plant.

An estimated 24 to 27 million gallons of wastewater also reached the river, according to the utility's initial estimates.

A revised calculation put the release down to 30,000 to 39,000 tons of ash, less than 4 percent of the total ash held in the basin, Duke said.

The drinking water of two towns in neighboring Virginia, Danville and South Boston, had to be treated to remove toxins, officials said.

In a subpoena dated February 10 to North Carolina's environmental department, prosecutors requested inspection records for the pipe dating back to 2010 and copies of correspondence between the agency and Duke Energy regarding the coal ash and wastewater discharge.

"The Department of Environment and Natural Resources will cooperate in this matter," said Drew Elliot, a spokesman for the agency.

The U.S. Attorney's Office was closed on Thursday due to a winter storm that blanketed the Southeast in snow and ice and officials were not available to comment.

The spill came amid a long-running legal battle over the storage of coal ash waste at Duke facilities in the state.

The North Carolina environmental regulator sued Duke last year over allegations of water quality violations at the company's plants. On Monday, the state asked a judge to put a controversial settlement deal on hold.

Environmental groups say the coal ash, a gray, powdery byproduct of power plants containing heavy metals that can cause cancer and nervous system damage, is stored in antiquated or unlined pits and risks seeping into groundwater and nearby rivers.

North Carolina public health officials have warned residents visiting the Dan River to avoid recreational contact with its water and sediment and not to eat any fish caught downstream from the spill site due to potential hazards from the coal ash.

The U.S. Environmental Protection Agency and state officials have said drinking water treated from the Dan River appeared safe.

(Editing by Grant McCool)

[Image via Duke Energy official Facebook page]