Gov Manchin says chances of finding surviving miners are slight
Rescue crews drilling holes into a stricken West Virginia coal mine have failed so far to detect any signs of life from four missing miners.
Search teams banging on a drill pipe to send a signal into the mine received no response, officials said.
Twenty-five miners are already known to have died after Monday's explosion at the Upper Big Branch mine.
Toxic gases have made it too dangerous for rescuers to enter the mine until it has been ventilated by drill holes.
The first drill hole broke through into the mine, about 30 miles (50 km) south of Charleston, on Wednesday morning.
Once the holes have been drilled through the top of the mine to ventilate a chamber more than 1,000 ft (305m) below ground, rescue crews will need four or five hours to reach the area where the miners are thought to be trapped, the chief operating officer for the mine's owner Massey Energy, Chris Adkins, told AP news agency.
"Given... the appearance of the explosion that was described by some of the rescue team members that I talked to, I don't give it much hope", Massey Energy chief Don Blankenship told West Virginia MetroNews Radio.
Conditions in the mine caused by the explosion, the worst US mining disaster for more than a quarter of a century, have made the rescue operation extremely difficult.
"There's so much dirt and dust and everything is so dark that it's very easy, as hard as it may seem to any of us outside in this room, to walk by a body," Mine Safety and Health Administration spokeman Kevin Stricklin was quoted as saying by AP.
Officials are not certain how long the drilling operation to vent the mine will take.
It could be midday on Wednesday (1600 GMT) before progress is made, West Virginia Governor Joe Manchin said.
The bodies of 14 miners remain inside the mine, after the build up of methane and carbon monoxide gases made it impossible to continue the rescue operation.
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