A lightning bolt travelled down a cable and ignited the methane blast that led to the deaths of 12 miners in West Virginia last year, a US report says.
The report, obtained by the Associated Press, also says methane in a sealed section was not monitored and that the seals did not contain the blast.
The blast sealed off the miners for 40 hours and only one, Randal McCloy, survived the carbon monoxide poisoning.
The disaster at the Sago mine led to sweeping changes in US mine safety.
Union report
The report says a number of possible causes were considered but that a lightning strike was deemed "the most likely ignition source", according to AP.
"We had to take off the rescuers [air packs] in order to hammer as hard as we could"
"Although a roof fall cannot be definitely excluded as a potential ignition source, it is a highly unlikely ignition source," the Mine Safety and Health Administration report says.
A mine workers' union which took part in the investigation had believed the deteriorating rock roof and metal supports had caused friction that ignited the blast on 2 January last year.
Previous reports by the mine owner and West Virginia state had indicated lightning was the most likely cause.
Production was halted at Sago in March on the grounds of high production costs and falling prices for the product.
The disaster led to allegations by the sole survivor that emergency oxygen bags were faulty.
But the mine's owner, International Coal Group, said the packs had been checked regularly.
Randal McCloy's allegations were made in a harrowing letter sent to the families of his late colleagues in which he described the desperate struggle for survival.
After attempts to hammer their way out failed and resigned to their fate, the men said a prayer, wrote farewell notes to loved ones and lost consciousness. Mr McCloy spent weeks in a coma.
The Sago disaster was the deadliest mining accident in West Virginia since November 1968, when 78 men - including the uncle of state Governor Joe Manchin, died in an explosion at a mine elsewhere in Upshur County.
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