The subpoenas to produce documents is related to a grand jury investigation apparently involving recordings of private conversations regarding the Pebble mine project.

Tom Collier, CEO of Pebble Limited Partnership, the company trying to develop Alaska’s Pebble Mine project, had resigned in September after his comments on elected and regulatory officials in the U.S. state were covertly videotaped and released by an environmental activist group.

The comments by Tom Collier had embellished both his and the company’s relationships with elected officials and federal representatives in Alaska, including Governor Michael Dunleavy, Senators Lisa Murkowski and Dan Sullivan and senior representatives of the U.S. Army Corps of Engineers (USACE).

The mine, one of the world’s largest copper and gold deposits, has been through a roller coaster of regulation over the past 13 years. Many opponents say the mine would permanently damage Alaska’s Bristol Bay watershed, an ecosystem that supports the world’s largest salmon runs.

The USACE in November issued its final decision denying a key water permit for the contentious Pebble mine, putting the project’s future in doubt.

Pebble Limited filed an appeal with the U.S. Army Corps of Engineers (USACE) over the decision, Northern Dynasty said in January.