West Virginia ? Nearly a year after the disaster, state investigators have concluded that the Sago Mine explosion that killed 12 miners was caused by a lightning bolt that ignited methane gas underground, a union official said.

The mine’s owner, International Coal Group Inc., has argued since March that lightning was to blame for the Jan. 2 blast, a theory critics of the company have disputed.

The report is to be released Monday, but United Mine Workers officials who helped in the investigation have been briefed on it, Dennis O’Dell, the union’s health and safety director, said Wednesday.

The chief of the state’s mine safety agency did not immediately return a call for comment.

ICG declined to comment.

One miner was killed in the blast, and 11 others died of carbon monoxide poisoning in the more than 40 hours it took rescuers to reach them. Only one trapped miner survived.