HUNTINGTON — Four hours of "quiet period" — with all operations at the Crandall Canyon Mine shut completely down — failed to reveal any clues as to the fate of six miners trapped deep underground here, rescue officials announced late Saturday.

"We listened by microphone and we also monitored through vibrations ... we did not detect any signals from miners underground," said assistant U.S. Labor Secretary Richard Stickler, who oversees the federal Mine Safety and Health Administration.

An 8 5/8-inch bore hole punched into the mine about 9:15 a.m. Saturday, and officials spent the next four hours listening for signs of life from the men, attempting to signal them by beating on the drill and setting off three rounds of explosives, Stickler said.

It is the fourth so-called bore hole to yield inconclusive results, though officials are still awaiting the analysis of videotape from cameras dropped into the mine on Saturday.

Still, the officials made clear during the late-night news conference that they have not given up on the idea of finding the six men alive.

"Make no mistake about it, this continues to be a rescue effort," said Rob Moore, vice president of Murray Energy Corp., which owns the Crandall Canyon Mine. "I want to make this clear. This continues to be a rescue effort. We have incurred setbacks, we have incurred losses, but we have and we will not give up hope."

Miners Kerry Allred, Don Erickson, Luis Hernandez, Carlos Payan, Brandon Phillips and Manuel Sanchez were two-thirds of the way through their 12-hour shift when the mine collapsed in the early morning hours of Aug. 6.

Meantime, officials announced they will soon begin work on a fifth bore hole while also assembling a team of mining experts to determine the feasibility of resuming underground rescue operations.

Those efforts were immediately suspended Thursday evening after a collapse killed three rescue workers and injured six others.

Stickler said experts from West Virginia University and the National Institute of Occupational Safety and Health would arrive throughout the night for an initial meeting at 8 a.m. today.

"These experts will review the conditions ... (and) make an assessment of those conditions to determine whether or not there's any reasonable way that we can resume the recovery work," he said.