HUNTINGTON, Utah — Six coal miners caught in a cave-in are probably dead and may forever be entombed in the still-quivering mountain, officials conceded yesterday, 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,” Mr. 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 August 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. Engineering experts gathered at the mine yesterday to try to figure out a way of reaching the men. Tunneling has been halted since Thursday’s deaths.

Mr. 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.”