SAN JUAN DE SABINAS, Mexico (AP) ­- Rescue workers arrived at the spot where two of 65 coal miners were believed trapped but found no sign of them, officials said Thursday.

Rescuers also were encountering increasingly toxic levels of methane gas as they advanced into the mine, an indication that they were less likely to find survivors, officials said. They stopped short of saying the miners were believed dead.

“The air as the rescue advances is increasingly lacking in oxygen and more laden with methane, which makes it less breathable,” federal Labor Secretary Francisco Salazar said.

Ruben Escudero, administrator of the Pasta de Conchos mine, told a news conference that rescuers had advanced 740 yards inside the mine, more than 110 yards beyond where two conveyor belt operators had been believed trapped.

“The conditions are becoming increasingly adverse,” Escudero said. “It is grave and, being realistic, we think the situation is difficult.” He declined to elaborate.

Rescuers had said that whether the two men were found alive or dead would indicate the condition of other workers.

Escudero said there was no sign yet of the two men, which he said meant they either were buried under debris or in a different part of the mine.

Mine owners and government officials have repeatedly refused to rule out the possibility of survivors more than four days after Sunday’s pre-dawn gas explosion.

Escudero said 72 workers were working around the clock to remove 600 to 800 tons of debris.

Miners interviewed by The Associated Press told of being sent deep into dangerously unstable shafts without training or proper equipment.

“They give you basic equipment and no training,” said Clemente Rivera, 28, a mine worker whose two cousins and a neighbor remained underground.

Its operators said the mine, in Coahuila state about 85 miles southwest of Eagle Pass, Tex., passed recent government inspections. Pedro Camarillo, a federal labor official, said nothing unusual was found during a routine inspection Feb. 7.

Juan Rebolledo, vice president of international affairs for mine owner Grupo Mexico, said that the mine met national and international safety standards.

“Accidents can always happen,” he said.

Rivera said he had been entering the mine with no more than rubber boots, a helmet with a lamp and an oxygen tank carrying an hour’s worth of air.

“Here you sign a contract, and the next day they put you in the mines without even a tour, or any training,” said Rivera, whose shift ended 12 hours before the explosion.

The national miners’ union said late Wednesday that miners had gone on strike 14 times partly because of the mine’s “constant refusal to review security and health measures.”

Relatives interviewed by the AP said that many of the 65 trapped coal miners may have carried less than an hour of oxygen, contradicting state officials who said they had a six-hour supply.

After four days of digging through hundreds of tons of rubble, weary rescue workers, many of them miners, said Thursday they would not give up the search.

“We are not going to abandon our comrades, dead or alive,” said Alvaro Cortes, his face lined with exhaustion and blackened with coal as he left the mine early Thursday after an overnight shift.

Four mining experts from the U.S. Mine Safety and Health Administration were at the mine with equipment for analyzing gas samples.

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