It could take up to two days to drill bore holes into a sprawling West Virginia coal mine, scene of a massive blast that left at least 25 miners dead, a mining official said Tuesday.

Bulldozers will be used to clear a path through the hills and bring in equipment to reach the affected part of the Upper Big Branch Mine, Kevin Stricklin of the U.S. Mine Safety and Health Administration said.

The holes will be drilled 1,200 feet down to help ventilate the mine and collect samples. Rescue crews early Tuesday halted their efforts to reach four miners still unaccounted for, as concentrations of methane and carbon monoxide posed a safety risk, Stricklin said.

"I think it's a dire situation but I do think that it is a rescue operation and it will be that way until we confirm that these four additional people are not living," he said earlier. "I mean, there are miracles that go on."

"Basically all we have left," Stricklin said, "is hope."

The explosion at the Massey Energy Co. mine, about 30 miles south of Charleston, West Virginia, took place during a shift change Monday afternoon.

Seven bodies have been brought out and identified. Among them were three members of the same family: an uncle and two nephews, said West Virginia's Gov. Joe Manchin.

Eighteen other bodies remained inside the mine, and four miners are missing, Manchin said.

Authorities do not yet know the cause of the explosion. But the mine has a troubled safety record, with three other deaths in the past 12 years, federal records show.

By early Tuesday, nine rescue teams had arrived on-site. Their goal was to race toward the mine's internal rescue chambers where miners are trained to seek refuge after an accident. However, the buildup of methane and carbon monoxide forced their withdrawal.

Such airtight chambers -- put in place following several deadly mining accidents in 2006 -- are stocked with enough food and water to enable workers to survive for four days.

Crews noticed that a number of breathing devices had been taken from storage areas inside the mine.

This, Stricklin said, gave officials hope that some of the miners who survived the initial explosion may have taken them to make breathing easier as they made their way to the chambers.

He could not say how many of the devices -- known as SCSR or self-contained self-rescue devices -- were taken.

But even as rescuers hoped to reach the chambers, the concentrations of methane and carbon monoxide that crews detected "were to the point that they were risking their own lives," Stricklin said.

"A decision was made at the time to evacuate the rescue teams from the mine," he said.

Distraught family members waited and prayed Monday in a Massey building, shielded from reporters.

"I told them, 'The good Lord didn't give me the words to comfort you,'" said Gov. Manchin. "I told them to do what they do best: love each other and come together as a family. "

Jenny Waycaster rushed to the mine when she heard about the explosion. Her son -- like her father -- is a miner. And the blast went off just as he was about to start his evening shift.

"He was one of the fortunate ones," she said. "I was blessed. I was blessed."

Soon afterward, she and her son started hearing from friends who had lost their sons and fathers.

"There's so many people hurting," Waycaster said. "This is a good place up here and it's family."

While the cause of the explosion was not immediately known, methane gas has been blamed in several deadly mining accidents in recent years.

They include the 2006 explosion at the Sago mine, also in West Virginia, that killed 12 people.

At the Upper Big Branch Mine, three other deaths have occurred in the past 12 years, according to federal records:

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