Stickler said rescuers would continue to drill bore holes through the top of the mountain in an attempt to find them.

Thursday's collapse was caused by a seismic tremor, known as a mountain bump, an eruption of rock and coal under increased pressure from overhead rock as drilling removes surrounding rock and material shifts in an area of the mine.

Stickler said the collapse also unleashed a blast of coal and support material that hit the miners working to clear rubble from the underground tunnel.

Destruction zone

The blast created a destruction zone about 9 metres long, knocking out steel posts, chain link fencing and the cables that tied everything together.

"When that energy gets released, it's like an explosion," he said.

Before Thursday's cave-in, rescuers still had about 366 metres to go before they reached the area where they believe the trapped men had been working.

They had advanced only 252 metres in nine days.

"Yesterday we went from a tragedy to a catastrophe," Jon Huntsman, Utah governor, said as he called for new efforts to make mining safer in his state and the country.

He said the state would help federal officials with the investigation into the collapse.

"We have questions, too, and we want answers to those questions. We want to make sure that the lives that were lost last night were not in vain."