A mining professor suggested that an electrical pulse from a lightning strike snaked into the mouth of Sago Mine and traveled down a conveyor belt, where it caused the explosion that led to the deaths of 12 miners.

Once inside, the charge stopped just feet from the sealed-off section where the blast occurred, Thomas Novak, a professor at Virginia Tech, told a panel Wednesday on the second day of hearings into what caused the blast.

"Lightning doesn't have to strike something directly" to cause an explosion, Novak said, then agreed under questioning that his preliminary findings could be characterized as a "hypothesis."

Many at the hearings have been skeptical about the explanation. International Coal Group, which owns the mine, announced in March that its internal investigation pointed to lightning as the cause of the blast, though it could not explain the precise conduit the electricity took into the mine.

The federal Mine Safety and Health Administration has ruled out several ignition sources, but not lightning or roof falls, said Richard Gates, the agency's investigation team leader.

An explosive concentration of methane gas "could easily be ignited by a weak electrical spark," as weak as the static electricity generated by walking across a carpet on a dry day, Gates testified Thursday.

The Jan. 2 blast killed one miner immediately, while 12 other miners slowly succumbed to carbon monoxide poisoning in the gas-filled mine. One miner, Randal McCloy Jr., survived.

United Mine Workers President Cecil Roberts challenged Novak on the improbability of the explosion having been caused by a lightning strike so far from the mine — and one traveling such an indirect path.

"In the history of coal mining in North America ... can you cite one single incident where lightning has struck the ground without going through a conduit of some type, such as metal pipe?" Roberts asked.

"No, I can't," Novak said.

"But you come today suggesting that that's what happened at Sago, is that correct?"

After a pause, Novak replied, "That's correct."

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

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

ICG said it plans to hire another expert to assess the mine geology for the possibility of metal in the rock walls, roof and floor of the sealed area.

Performance of the seals on a worked-out section of the mine is another area investigators have focused on as they explore the causes and aftermath of the explosion that killed 12 miners. Investigators and officials with ICG believe methane had accumulated near the seals and was somehow ignited.

Dense foam block seals were blown apart by forces as much as 4 1/2 times stronger than they were built to withstand, Stephen G. Sawyer, a former engineer with the Mine Safety and Health Administration hired as a consultant by ICG, testified.

Every seal involved was exposed to at least 25 pounds of pressure per square inch, slightly higher than the 20 psi federal standard for sealed mines, Sawyer said. What remains unclear is the amount of pressure it takes for the seals to fail and whether federal regulations should be toughened.

Participants, including lawmakers and miners' families, also challenged Sawyer, forcing him to concede that while he'd been underground many times, Sago was his first trip into a mine's worked-out, sealed-off part.

"I bet it was for a lot of people," Sawyer said. "One thing about Sago: We're going to learn a lot."