(Recasts with drilling taking longer than expected)

By James Nelson

HUNTINGTON, Utah, Aug 9 (Reuters) - Rescue crews who had hoped to reach six miners trapped deep in a collapsed Utah mine on Thursday instead found themselves working well into the night, frustrated as drilling from the surface took hours longer than expected.

Family members gathered at a school near the Crandall Canyon mine on Thursday afternoon expecting word on the rescue operation, which mine co-owner Robert Murray had predicted would reach the stranded men before dark.

Instead they were told that the drilling had gone more slowly than expected through hard rock and sandstone and that it would take several more hours to reach the men -- who have not been heard from since the mine caved in on Monday morning and could already be dead.

"We'll know in a matter of hours if we've hit the cavity where the men are, or a block of solid coal," Murray told CNN on Thursday evening, referring to the possibility that the drill could miss its intended target.

Murray said earlier that the 2 1/2-inch (6 cm) drill had reached 1,730 feet (527 meters), putting it about 100 feet from the target area. At its pace of 20 feet per hour, it would reach the area in about five hours.

He gave a different figure in his interview with CNN, however, saying the drill was down to 1,570 feet (478 meters).

Once the drill pierces the cavity where the men are thought to be stuck, it will take another two hours for communication equipment to be lowered through the hole and determine whether they are alive, Murray said.

MINE OWNER STILL 'OPTIMISTIC'

"Within two hours of the time that the drill hole breaks through into the cavity, the two and a half inch hole, we will drop down communication devices. And we could know then," he said.

The bore hole could also be used to provide food, water, and air to the men while a larger opening is created to get them out, which could take another week.

A second bore hole, nearly 9 inches (23 cm) in diameter, is being drilled and should reach the section where the men are believed to be late on Friday, according to Murray.

Officials say the miners could survive for weeks in an underground chamber.

"I will say that I'm optimistic," Murray told CNN. "There's good ventilation where the recovery effort is going on and plenty of water available to them. If the men were not killed from original earthquake and their bodies were not destroyed then there is a very good chance they still alive.

Murray has insisted that an earthquake triggered the mine's collapse but geologists dispute that, saying that shaking recorded by their instruments was caused by the cave-in.

Controversy has also risen over reports that the miners were engaged in a dangerous operation called "retreat mining" when the shaft collapsed -- though Murray has denied that such a technique was being used.

Retreat mining involves supporting the mine's roof with a column of coal, then removing those pillars and allowing the shaft to collapse as miners move to safety.

The Crandall Canyon Mine is on a high desert plateau some 140 miles (225 km) south of Salt Lake City, in what is known as Utah's "castle country" because of the towering rock spires that dot the bleak landscape. (Additional reporting by Dan Whitcomb in Los Angeles and Eric Beech in Washington)

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