The Mine Safety Health Administration is blaming managers for not preventing an underground avalanche that killed a contract worker at the Lucky Friday mine.
Brandon Gray, 26, suffocated when he was engulfed in a rock slide inside a storage bin for excavated ore on November 17, 2011.
Gray and another worker were trying to dislodge rubble that had clogged a chute that would have allowed the broken rock to flow into a lower level of the mine.
Both men were wearing self-retracting safety harnesses designed to recoil and keep the men from falling, but MSHA officials say the flowing rock was descending too slowly to activate the harnesses.
The safety gear allows miners to move around freely unless they experience a sudden drop and are similar to seatbelts in cars.
Investigators say the men should have been secured by a fixed tie-off line monitored by another miner.
The Lucky Friday and its contractor, Cementation U.S.A , have agreed to provide more training and not allow workers to stand on top of ore that has the potential to move.