The explosion that ripped through a West Virginia mine last January, leaving 12 miners dead, was nearly five times more powerful than the mine's underground seals were able to withstand, a state report says.

The methane gas explosion, likely caused by lightning, was in an abandoned section of the Sago Mine that had been sealed less than a month earlier.

The seals were designed by federal standards to withstand forces of 20 pounds per square inch, but state investigators found 10 seals were blown apart by forces at least 95 pounds per square inch.

"At this writing, there is reason to suspect that explosion pressures in excess of 100 psi may have been developed," the state Office of Miners' Health, Safety and Training says in the report, obtained by the Associated Press.

Six months after the deadly Jan. 2 blast, the U.S. Mine Safety and Health Administration ordered that all seals must now withstand 50 psi — still less than the force of the Sago blast.

The report was to be released Monday, but it was delayed in response to requests from the families for additional information.

The cause of the explosion is clearly related to lightning, the report says, but "how the electricity from the lightning entered the sealed area is still under investigation, and in that regard this report is not complete."

The explosion occurred about 6:30 a.m., trapping 13 miners who had gone underground to resume production after the holiday.

Only one miner survived. One was killed in the explosion and 11 died of carbon monoxide poisoning while awaiting rescue for more than 40 hours.

After the blast at the International Coal Group-owned mine, families were told 11 miners had survived. Hours later, they were told initial reports were incorrect.

Monday's report faulted the emergency air packs miners carry with them, saying the packs "did not perform in the manner expected."

State investigators suggested more answers might be forthcoming from the Mine Safety and Health Administration.

The U.S. government agency has hired investigators to determine if electricity can travel through equipment such as the metal conveyer belt or simply move through solid ground. Test results are expected next year.