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; a U.S. Geological Survey station confirmed a seismic event at Sago; and the mine's own atmospheric alarms sounded.
The precise route the electrical charge followed remains under investigation, but Hatfield said there is no evidence a nearby gas well contributed to the explosion.
The company's announcement provides the strongest indication so far of what may have caused the blast. Since the accident, state and federal regulators have declined to speculate, but lightning was suggested as a possible cause on the day of the disaster because severe thunderstorms moved through the area at the time.
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."
LuluBelle Jones, whose son Jesse died at Sago, said the families knew lightning was a possible cause and were not surprised by the news. Still, the meetings were difficult.
"There was just a lot of crying and stuff," she said. "It's still hard on us."
A spokeswoman for Gov. Joe Manchin said it would be premature to discuss the findings until a state team conducts public hearings and submits a final report later this year.
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. On Tuesday, he visited his home for the first time since the explosion, eating a home-cooked lunch, and later returned to a rehabilitation hospital.
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.
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."
Ashland, Ky.-based ICG has aided state and federal officials throughout the Sago probe but also hired outside consultants to do an independent investigation.
Hatfield said the team concluded the explosion occurred behind 40-inch-thick, dense foam block seals that were designed to withstand 20 pounds per square inch of pressure. The seals, called Omega blocks, were pulverized by a blast that initial calculations suggest had forces of at least three times that amount in some locations.
Hatfield told The Associated Press on Tuesday that ICG will no longer install seals at the Sago Mine and will ventilate existing sealed areas with boreholes to the surface. In time, the company hopes to adopt a technology used in some other mining countries — pumping nitrogen into the abandoned areas to make them inert.
"That's something we're going to take a hard look at," he said, adding that the inert gas technology also could be used at other ICG mines.
Historically, the industry has believed seals were the safest way to manage abandoned areas, Hatfield said. When the areas are not walled off, a fire boss must be sent in periodically to confirm it is being ventilated and deadly or explosive gases are not gathering.
Hatfield also said Sago will comply as quickly as possible with federal regulations enacted last week. Companies now must give each miner an extra air pack and, in some cases, store additional breathing devices along escape routes.
Hatfield said ICG has obtained enough air packs to comply but is still working on meeting the lifeline requirement.
ICG bought the Sago Mine near Buckhannon from bankrupt Anker West Virginia Mining Co. last March. The operation has been producing coal since September 1999 and had 145 employees at the time of the accident.