Piedras Negras, Mexico ? A gas buildup in a northern Mexico coal mine triggered a pre-dawn explosion Sunday, trapping at least 65 coal miners underground with a limited supply of oxygen. Emergency officials were tunneling through the debris to rescue them.

At least eight miners who had been near the mine’s exit when the explosion occurred were rescued and were hospitalized with burns and broken bones.

Union and company officials said they believed there were 65 miners trapped in various places throughout the mine, near the town of Sabinas, 85 miles southwest of Eagle Pass, Texas. Rescue officials had not been able to make contact with the trapped miners.

Juan Rebolledo, vice president of international affairs for mining giant Grupo Mexico, which owns the mine, said several rescue teams were taking turns carefully removing debris that had clogged the steep shaft.

“It’s slow work because of the quantity of debris,” he said.

Rebolledo said officials were unaware of the miners’ conditions. He said oxygen tanks were scattered throughout the mine, but it was impossible to know whether the trapped miners had access to them or how long they could survive before rescue workers reached them. The explosion occurred around 2:30 a.m. Sunday local time.

Coahuila Gov. Humberto Moreira Valdes, who was at the site overseeing the rescue operation, told Televisa network that the mine’s ventilation system was still working. The mine was about 985 feet below the ground.

Consuelo Aguilar, a spokeswoman for the National Miners’ Union, said union officials were also at the scene.

She said there has been concern over safety conditions in Grupo Mexico mines, and called for an investigation to determine the cause of the accident and the responsibility of any company officials.

Rebolledo said safety conditions at the mine met Mexican government requirements as well as international standards.

“We follow all the best safety procedures, but accidents can always happen,” he said.