Governor Joe Manchin ordered flags lowered to half staff in West Virginia to mourn the deaths of at least 25 miners in an explosion at the Upper Big Branch Mine in Montcoal on Monday.
Rescuers are drilling four holes as deep as 1200 feet to remove toxic gases, so search teams can enter and try to find the four men not confirmed dead.
Samples taken Wednesday morning showed unsafe levels of carbon monoxide and hydrogen.
According to news reports, rescuers have not heard any sounds from the four missing miners.
Kevin Stricklin, an official with the federal Mine Safety and Health Administration, said at a news conference late Wednesday afternoon that it was still too dangerous for rescuers to enter the mine.