RALEIGH, N.C. (AP) — A federal grand jury was set to convene Tuesday as part of a widening criminal investigation triggered by the massive Duke Energy coal ash spill that coated 70 miles of the Dan River with toxic sludge.

The session at the federal courthouse in Raleigh comes as environmental groups amp up pressure on regulators and lawmakers to force Duke to clean up the leaky, unlined ash pits polluting North Carolina’s waterways. Prosecutors have issued at least 23 grand jury subpoenas to Duke executives and state officials.