Amy Adams, North Carolina campaign coordinator with Appalachian Voices, shows her hand covered with wet coal ash from the Dan River swirling in the background as state and federal environmental officials continued their investigations of a spill of coal ash into the river in Danville, Va., Wednesday, Feb. 5, 2014. Duke Energy estimates that up to 82,000 tons of ash has been released from a break in a 48-inch storm water pipe at the Dan River Power Plant in Eden N.C. on Sunday. (AP Photo/Gerry Broome)

Federal authorities have initiated an investigation into the North Carolina Department of Environment and Natural Resources following last week's coal ash spill in the Dan River.

The Associated Press first obtained a copy of a grand jury subpoena that the U.S. attorney’s office in Raleigh issued requesting emails, memos and reports from the department dating back to 2010. The subpoena compels counsel for the DENR to appear before a grand jury March 18 to 20.

DENR spokesman Drew Elliot would not comment on the subpoena in a statement to The Huffington Post, beyond stating that, "The Department of Environment and Natural Resources will cooperate in this matter."

Pressure has increased on the state following the coal ash spill from a retired Duke Energy facility in Eden, N.C. Initial estimates put the spill at as much as 82,000 tons, but on Thursday Duke reduced its estimate to between 30,000 and 39,000 tons of coal ash. Environmental groups in the state say that, rather than enforcing environmental laws, the DENR has protected Duke for years -- thereby allowing a spill like this to happen.

"It's about time someone took a hard look at the disturbingly cozy relationship ... ," said Gabriel Wisniewski, energy campaign director at Greenpeace US. "The need for a federal investigation is sad, but not surprising."

The Waterkeeper Alliance and Yadkin Riverkeeper issued a statement Thursday saying that their independent testing of the Dan River showed that coal ash is still leaking into the river from a site about a third of a mile upstream from where the pipe broke. Duke Energy spokeswoman Lisa Hoffmann said that the area they tested is near a storm drain from another part of the plant and is "not associated with the ash basin."