The company's own investigation turned up three pieces of compelling evidence of a lightning strike, all from 6:26 a.m. on Jan. 2, said Ben Hatfield, chief executive officer of International Coal Group Inc.
He said weather monitors confirmed an unusually large and powerful lightning strike near the mine; the U.S. Geological Survey confirmed a seismic event at Sago; and the mine's own atmospheric monitoring system signaled a combustion alarm.
The precise route the electrical charge followed remains under investigation, but Hatfield said there is no evidence that a nearby gas well contributed to the explosion.
Hatfield broke the news to miners' families in a series of private meetings Tuesday, and Sago workers were to be briefed Tuesday night as they returned to work. The coal mine is set to resume production Wednesday.
"While our independent investigation is certainly not the final word on the explosion, we are confident that the joint federal-state investigation will reach a similar conclusion," Hatfield said. "We are pleased that we can get our Sago employees back to work with the knowledge that the explosion was an unpredictable and highly unusual accident."
The explosion trapped a crew of 13 men more than 250 feet underground for more than 40 hours. By the time rescue teams reached them, all but one had perished, most slowly succumbing to carbon monoxide poisoning. Survivor Randal L. McCloy Jr. is still recovering from severe brain damage and other injuries.
Although the federal Mine Safety and Health Administration declared the mine safe to re-enter last week, Hatfield said he delayed resuming production until he could share the initial findings with the families.
Hatfield said reactions ranged from anger and frustration to relief. Mainly, though, families appreciated getting the information before it was released to the media or the general public.
He said he promised the families that lessons will be learned from the disaster, and coal mines will be made safer.
Though MSHA cited the mine for 208 violations in the months before the accident, the company's investigation showed that none of those violations was related to the blast, Hatfield said. Still, the company expects to be under the microscope.
"Frankly, we welcome that scrutiny," he said. "We have worked hard to address all concerns and are confident that we will provide a safe working environment for our miners."