State investigators have determined that a lightning strike sparked the methane gas explosion deep inside the Sago Mine, where 12 coal miners died in January, a union official said.

Lightning had been suspected from the beginning, and a nearly yearlong state investigation ruled out other potential causes for the explosion, said Dennis O'Dell, the labor union's health and safety coordinator.

One miner was killed by the Jan. 2 blast, and 12 others were trapped underground for 41 hours. When rescuers reached them, only one man was still alive. The others had succumbed to carbon monoxide poisoning.

The state Office of Miners' Health, Safety and Training plans to release its report on the Sago Mine disaster on Monday, but office director Ron Wooten spoke about the report's conclusions last week to several members of the United Mine Workers, O'Dell told The Associated Press.

Other people familiar with both the report and the investigation confirmed that the state had ruled out other possibilities, such as a roof fall, and would lay out circumstantial evidence for a lightning strike.

They spoke on condition of anonymity because the report has not been made public and Gov. Joe Manchin plans to brief the families of the dead miners before its release.

Although rare, lightning strikes have been known to cause underground explosions.

The National Institute for Occupational Safety and Health blamed lightning for seven coal mine explosions that occurred in sealed, abandoned areas in a report issued in 2001. In most cases, the seals on worked-out sections were destroyed, and in all, some metallic conduit was later identified.

The Sago Mine's owner, Scott Depot-based International Coal Group, has also blamed lightning for the explosion, and cited three pieces of evidence: the presence of a powerful lightning strike near the mine, a U.S. Geologic Survey station's confirmation of a seismic event at the mine, and the sounding of the mine's own atmospheric alarms.

A spokesman declined to comment Thursday on the state's conclusion, saying the company preferred to wait until the report is released.

An interim report issued by Manchin's special adviser, former federal Mine Safety and Health Administration chief J. Davitt McAteer, earlier this year questioned if ICG took all required steps to try to protect the mine against lightning strikes. He said ICG failed to properly ground the mine's electrical systems.

The explosion remains the subject of a federal probe.

The sole survivor among the miners, Randal McCloy Jr., has said four members of the trapped crew could not get their emergency air packs to work, thwarting their ability to escape in thick smoke through a jumble of damaged mine equipment. They erected a makeshift barricade 260 feet below the surface and waited for rescuers. People familiar with the investigation say the report also addresses how the air packs worked.

In the 11 months since the Sago explosion, state and federal mining regulators have adopted numerous new rules and laws. Chief among them are measures requiring mine operators to keep stockpiles of extra air packs underground to supplement devices carried by individual miners at most mines.

For the U.S. coal mining industry, 2006 has become its deadliest year in over a decade, with 46 miners killed on the job, including 23 in West Virginia.