A rock burst at Hecla’s Lucky Friday silver mine in northern Idaho injured three miners, but all 20 miners were rescued from the mine following the burst on Dec. 14.

The miners were working about 1.800 m (5,900 ft) underground when they were by a rock burst. The Lucky Friday Mine is, one of the nation’s deepest underground mines. It is also the site of two fatalities in 2010.

The federal Mine Safety and Health Administration will investigate the accident that happened months after two other Lucky Friday silver miners died.

In April, a roof collapse in a tunnel more than a mile underground trapped Larry Marek. Crews recovered his body nine days later, The Associated Press reported.

Last month, Brandon Gray was buried in rubble after trying to dislodge a jammed rock bin. He died from his injuries two days later.

Shortly after Gray died, Mine Safety and Health Administration regulators criticized Hecla for safety failures that led to Marek's death. The mine received four citations and faces nearly $1 million in penalties, the Spokesman-Review of Spokane, Wash., reported.

The investigation report cited Lucky Friday management for failing to install adequate ground support systems and neglecting to test the stability of the area where the collapse that killed Marek occurred.

The mine is currently undergoing a $200 million project to deepen it to nearly 2,700 m (9,000 ft) to increase access to deeper silver deposits. Hecla officials expect the project to be completed by 2014.

“We are thankful that all employees are out of the mine and have been accounted for, and that those injured have been treated. The safety of our employees is our primary concern,” said Phil Baker, president and chief executive officer. “The mine is currently shut down, and once we have cared for our people, we will be investigating the cause of the seismic activity.”

Seven people were transported to local hospitals and treated for non-life-threatening injuries. No mine blasting had taken place anywhere in the mine for the previous 24 hours; therefore, the rock burst is unrelated to mining activities. The mine was  closed pending further investigation.