MONTCOAL, W.Va. - An explosion rocked a remote coal mine with a history of safety problems Monday, killing 12 workers and trapping 10 others thousands of feet underground.

Rescuers early today were making their way to the area the workers were believed trapped at Massey Energy Co.'s sprawling Upper Big Branch mine, where the blast occurred around 3 p.m., said Kevin Stricklin, an administrator for the federal Mine Safety and Health Administration.

"It's important for us to try to get to the survivors as quickly as possible," said Stricklin.

He said officials hope the miners survived the initial blast and were able to reach airtight chambers that are stocked with food, water and enough oxygen for them to live for four days.

Though the cause of the blast was not known, the operation about 30 miles south of Charleston has a history of violations for not properly ventilating highly combustible methane gas, safety officials said.

Miners were leaving on a vehicle that takes them in and out of the long shaft, when a crew ahead of the them felt a blast of air and went back to investigate, Stricklin said previously.

They found nine workers, seven of whom were dead. Early today he raised the death toll to 12. Two others were injured. Two other nine-person crews and a safety inspector who had been working alone were believed trapped, perhaps about a mile and a half underground, said Stricklin, an administrator for Coal Mine Safety and Health. Officials do not believe that the roof collapsed.

"We want to assure the families of all the miners we are taking every action possible to locate and rescue those still missing," said Massey CEO Don Blankenship.

Rescuers found evidence suggesting miners took emergency oxygen supplies taken from a cache in the mine, Stricklin said. Federal and state regulators have required mine operators to store extra oxygen supplies in underground mines since 2006. The mine uses containers that can generate about an hour of breathable air. All miners carry a container on their belt besides the stockpiles in the mine.

Distraught family members were briefed and taken to a Massey building off-limits to the media.

MSHA officials didn't yet know what caused the blast, but federal records say the Eagle coal seam releases up to 2 million cubic feet of methane gas into the mine every 24 hours, which is a large amount, said Dennis O'Dell, health and safety director for the United Mine Workers labor union.

Methane is one of the great dangers of coal mining. The colorless, odorless gas is often sold to American consumers to heat homes and cook meals. In mines, giant fans are used to keep methane concentrations below certain levels. In 2006, 12 miners died in a methane explosion at the Sago Mine in West Virginia. If concentrations are allowed to reach between 5 percent and 15 percent, the gas can explode with a spark roughly similar to the static charge created by walking across a carpet in winter.

In the past year, federal inspectors have cited Massey and fined the company more than $382,000 for repeated serious violations involving its ventilation plan and equipment at the mine run by subsidiary Performance Coal Co. The violations also cover failing to follow the plan, allowing combustible coal dust to pile up, and having improper firefighting equipment.