As frustration mounts over the slow pace of the digging to free six trapped miners, more questions arose Tuesday about whether risky mining methods may have left parts of the coal mine dangerously unstable.

Some mining companies consider the "retreat mining" methods used at Utah's Crandall Canyon so dangerous, they will leave behind coal rather than risk the safety of their workers.

Video images taken early Tuesday showed miners working to clear a heavily damaged mine shaft. They were only a third of the way to the presumed location of the trapped miners -- eight days after a thunderous collapse blew out the walls of mine shafts.

A top mining executive estimated the digging would take up to another week.

"It's not fast enough for me," said Bob Murray, chief of Murray Energy Corp., co-owner and operator of the Crandall Canyon mine. "It's very painful."

Much of the rescuers' time is spent shoring up walls and ceilings before a 65-ton machine can resume clawing away at the rubble-filled mine shaft.

"We're doing the very best we can as fast as we can," said Richard Stickler, head of the federal Mine Safety and Health Administration. "You couldn't get another person into that working area."

Around the clock, shifts of 80 miners are digging and helping to remove the rubble.

Above ground, crews drilling another camera hole were about halfway to breaking into a rear section of the mine where they believed the men may have taken refuge in an air pocket. Murray said it could take another day for the drilling to break through.

A second 8-inch drill hole is being used to pump fresh air into the mine. Officials are taking air samples from a smaller hole at 2 1/2 inches.

The mine may have been made more dangerous by what Murray acknowledged was decades of digging using retreat mining, a common though sometimes dangerous method in which miners yank out a mine's pillars, grabbing the last of the coal.

Murray said the retreat mining took place before he took over the mine a year ago. He said no retreat mining was taking place at the time of the collapse, which he insists was triggered by an earthquake. Government seismologists say the mine's collapse registered as an earthquake.

"There's no connection between retreat mining and the natural disaster that occurred here," Murray said Tuesday. "I've said that from the beginning, and that's the way it will eventually come out."

Mine-safety experts say that two sections of the Crandall Canyon Mine that collapsed in March may have been an early warning sign. They questioned whether the company -- and the government agency that oversees its work -- should have closed the mine then.

Instead, operators moved to another section and continued chipping away at the coal.

"Knowing all the issues, they made a conscious decision" to keep mining "because they wanted to recover that coal," said Tony Oppegard, a former top federal and state of Kentucky mine safety official who represents miners as a private attorney in Lexington, Ky.

The experts now think Crandall Canyon was particularly unstable because of a combination of factors.

The section the miners were working was being carved out in a pattern like streets on a city block, leaving pillars to hold up the ceiling. Officials at the Mine Safety and Health Administration say they had approved a plan to allow "retreat" mining there.

But experts question that decision because the area is bordered by two outer sections that had already been mined and collapsed, using a technique that leaves behind unstable rubble.

That means the last pillars were bearing much of the weight of the roughly 2,000 feet of mountain above, and as they were pulled down, the pressure on the remaining pillars would have increased.

Larry Grayson, who worked in coal mining for nine years until 1984 and is now a professor of energy and mineral engineering at Pennsylvania State University, said retreat mining is so risky that the mining company he worked for would not use it between two sections of rubble.

The mining strategy at Crandall Canyon just didn't work, he said. "There was no advance notice, and just -- wham-o."

Murray has said that federal regulators and an outside mining engineering firm had signed off on the canyons' mining operation.