An explosion in a remote section of a coal mine three miles underground sent a fireball through a shaft to where miners were working, injuring 11 of them, officials said. Nine remained hospitalized Thursday, four with serious injuries.

A second blast 50 minutes after the first caused no injuries.Investigators worked through the night deep in the Golden Eagle coal mine to make sure gases that caused the explosions early Wednesday were being vented properly, said Kathy Snyder, spokeswoman for the federal Mines Safety and Health Administration.