All the injured miners were expected to fully recover . The two most serious injuries were a broken arm and a broken pelvis.

The mine remained closed and the federal Mine Safety and Health Administration is investigating, the Associated Press reported.

The latest rock burst was unrelated to two separate accidents earlier this year that killed two miners at the Lucky Friday, Hecla president and chief executive officer Phil Baker said.

Hecla said that on Nov. 16, a small seismic event triggered by blasting in the mine caused a rock burst at roughly the same location as the latest accident 5,900 feet below the surface of the silver mine. No one was injured in that incident, Hecla said.

A rock burst is a common mining accident in which excessive pressure causes a rock to fracture or explode.

There was no blasting in the mine for 24 hours prior to Wednesday's rock burst, Hecla said.

That raised the possibility that a seismic event alone caused the burst.