TALLMANSVILLE, W.Va. - Rescuers pushed deeper into a mine shaft in a desperate search for 13 trapped coal miners Tuesday, but the prospects of finding anyone alive appeared bleak after holes drilled into the ground yielded deadly levels of carbon monoxide and no signs of life

"With each hour that passes, the likelihood of a successful outcome diminishes," said Ben Hatfield, chief executive of mine owner International Coal Group Inc.

By early evening, Hatfield said, rescuers were three to five hours from reaching the spot where the miners were thought to be.

"We are clearly in the situation where we need a miracle," he said. "But miracles happen."

The men, trapped 260 feet down by an explosion Monday morning in the Sago Mine, were believed to be about 12,000 feet past the opening of the shaft. By early evening, about 35 hours after the blast, rescue teams had penetrated 11,400 feet, working their way on foot for fear machinery might cause volatile gases to explode.

The company told families that a powerful explosion had rocked the mine, based on damage near where the miners may be trapped, said Rick McGee, who works at the mine with his brother-in-law, Randal McCloy, who is among those trapped.

Cinderblock walls meant to direct the flow of air inside the mine were knocked down by the blast, McGee said.

Given the new information, McGee said, "There's a chance, not a great chance, but there is still a chance" that the miners could still be alive if they were able to barricade themselves.

President Bush said the nation was praying for the men, and he offered federal help to bring them out alive. "May God bless those who are trapped below the earth," he said.

Earlier in the day, rescuers drilled narrow holes into the mine, inserted air monitors and found levels of carbon monoxide more than three times the maximum regarded as safe. Carbon monoxide, a byproduct of combustion, can be lethal.

Hatfield said it was possible the miners barricaded themselves somewhere and were still alive. But he said: "We are very discouraged by the results of this test."

Also, a camera lowered down a 6 1/4-inch hole spotted no sign of the miners, and drilling crews pounded on a steel pipe and listened for a response but heard nothing, Hatfield said.

Some relatives said they were beginning to give up hope.

John Groves, 43, of Cleveland, W.Va., brother of trapped miner Jerry Groves, 57, said relatives were thinking the worst after Gov. Joe Manchin spoke with families Tuesday evening.

"He said the odds are against us. It's not hopeful," Groves said as he fought back tears. "I think everybody knows based on what they told us, what we are looking at."

After hearing the bleak assessments, many family members retreated to the nearby Sago Baptist Church. Hundreds had spent the night huddled in tents or wrapped in blankets in the cool, damp air.

"That don't sound good," said a red-eyed Donald Marsh, who kept an all-night vigil across from the mine where his half-brother, Jim Bennett, was trapped. Still, Marsh said he was unwilling to let go of all hope.

"Obviously, it was devastating," said Nick Helms, whose 50-year-old father, Terry, was among the missing. But Helms said his father once told him that mine air tests could be deceiving because safer air could be just a short distance away.

"My father and every person who goes into that mine knows what they're doing. I'm sure they found a way to stay safe," he said. "I just want to see him again."

At first, rescue crews moved slowly through the shaft, because they had to stabilize it and repair the roof as they went along. But on Tuesday, officials said, the rescuers realized they had no time to waste and abandoned that approach.

The Mine Safety and Health Administration had rescue and safety specialists on the scene, set up a command center and brought in a robot capable of exploring areas too dangerous for humans. But the robot was abandoned after it became bogged down in the mud.

The governor urged West Virginians to "believe in miracles."

"It's going to take a miracle, I think," he said.

The cause of the explosion about 100 miles northeast of Charleston was under investigation. Coal mine explosions are typically caused by buildups of naturally occurring methane gas. Manchin spokeswoman Lara Ramsburg said earlier that it may have been sparked by lightning.