Montcoal, West Virginia (CNN) - The death toll from the massive explosion at a sprawling coal mine in West Virginia rose to 25 early Tuesday, making it the deadliest U.S. mining disaster in 25 years.
Crews halted their efforts to reach four miners still unaccounted for at the Upper Big Branch Mine following the blast Monday afternoon.
Concentrations of methane and carbon monoxide inside the mine made it a safety risk for crews to proceed, said Kevin Stricklin of the U.S. Mine Safety and Health Administration at a 2 a.m. briefing to reporters.
Officials planned to drill bore holes from the surface 1,200 feet into the mines to help ventilate it and to collect samples. However, they will first have to use bulldozers to clear a path to reach the part of the mine where they can drill.
Stricklin said he did not know how long the process would take.
"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. "I mean, there are miracles that go on."
"Basically all we have left," Stricklin said, "is hope."