Six coal miners caught in a cave-in are probably dead and may forever be entombed in the still-quivering mountain, officials conceded Sunday, all but abandoning the unflinching optimism they've maintained publicly for nearly two weeks.

Air readings from a fourth hole drilled more than 1,500 feet into the mountainside found insufficient oxygen to support life, said Rob Moore, vice president of Murray Energy Corp., co-owner of the Crandall Canyon Mine.

"It's likely these miners may not be found," Moore said, expressing a marked shift in tone in his mine officials' assessments of the chances the men would be rescued, hopes they had maintained even after three rescuers were killed and six more hurt Thursday in another "bump" inside the mountain.

There has been little evidence that the six miners survived the initial Aug. 6 collapse. Workers have gained limited access to the mine through four boreholes into which video cameras and microphones were lowered. Rescuers banged on a drill bit and set off explosives Saturday, hoping to elicit a response from the men, yet their efforts were again met with silence.

Video images taken from the fourth hole showed signs of collapse in the cavern but no indication the miners were there, said Richard Stickler, head of the federal Mining Safety and Health Administration. Three previous efforts to reach the men via drilling have proved futile.

Engineering experts from around the nation gathered at the mine Sunday to try to figure out a safe way of reaching the missing men. Underground tunneling has been halted since Thursday's deaths.

Moore expressed doubt that the tunneling effort would resume.

"Thursday night we had an awful tragedy here," he said. "I can't say with certainty we will be able to continue the underground efforts. The risk is too great. We just simply cannot take the unacceptable risk and put additional lives in harm's way."

Moore had been far more upbeat Saturday night, when he insisted the men may be alive. But he said oxygen readings and video images taken from the fourth hole had changed his mind about the miners' probable fate. Oxygen levels in the hole are just 11 to 12 percent, incompatible with life. Normal oxygen levels are 21 percent.

Workers were starting Sunday on a fifth borehole into the mountain, more than 2,000 feet down, in another effort to find the men, but Moore said he expected to find insufficient air in that hole as well.

"Our thoughts and our prayers and our deepest sympathies go out to the families — for all those families involved in the two tragedies here," he said.

Moore met with family members Sunday morning for what he called a "very difficult, very emotional" discussion. He said their response was one of frustration, not anger.

"They're responding as I'm sure any of us would respond. They want to see their loved ones again. I want to be able to provide that but I can't give any guarantees we're going to be able to," he said.

MSHA summoned experts from the National Institute of Occupational Safety and Health, West Virginia University and private engineering firms in the hope that they can develop a safer way of tunneling toward the trapped miners. Their first meeting at the mine started Sunday morning.

The challenge is daunting. No support system currently in existence can withstand the explosive force of a mountain bump because those forces are nearly impossible to predict, Stickler said. Once one coal pillar collapses, the weight it had carried gets transferred to adjacent coal pillars, setting off a chain reaction.

"The mountain continues to be active, continues to move," Stickler said Sunday. "As the weight causes pillar failures in one area of the mine, then that weight is shifted to adjacent pillars and that process seems to be migrating out from the original area where the bump activity started."

There were "four or five" significant seismic events on the mountain Saturday, Stickler said.

The experts were studying mine maps and planned to go underground, into a part of the mine deemed safe, to examine the coal pillars holding up the roof.

If tunneling doesn't restart, part of the mine will have been turned into a tomb. Despite that, Moore said there is recoverable coal in other parts of the 5,000-acre mine, and the company expected to resume operations at some point. He said he didn't discuss that prospect with family members.