Two mine safety teams completed the first cautious exploration of West Virginia's Upper Big Branch coal mine in nearly two months Wednesday, and they hope to go back underground Friday.
The teams were initially forced to retreat temporarily when monitors indicated the presence of toxic and explosive gases.
"Both teams advanced approximately 1,000 feet but were forced to exit the mine ... after hand-held readings indicated potentially elevated levels of carbon monoxide and methane," said Amy Louviere, a spokeswoman for the federal Mine Safety and Health Administration. "Bottle samples were taken to confirm the hand-held readings."
The safety crews waited about four hours for analysis of the bottled air samples, which indicated that the gas levels were not as threatening as believed. They then went back underground through two separate entry points and worked their way toward each other, stringing a phone line along the way.
This exploration of the mine is part of an effort to kick-start a stalled investigation of the nation's deadliest coal mine disaster in 40 years. Twenty-nine mine workers died in a massive explosion April 5. Investigators have been unable to search for a cause for the blast because of methane gas and carbon monoxide in the mine. The methane gas is potentially explosive, and the carbon monoxide is an indicator of a smoldering fire or other heat source.
Mine owner Massey Energy proposes to re-enter the mine Friday, Louviere said, and to send the safety crews to the area believed to be the source of the fire or heat. That's at a long-wall mining machine that some miners and mine safety experts consider a possible ignition source for the explosion.
It may take two weeks to fully explore the mine, sample air and make sure there's enough air flow for safe entry by investigators, said Leslie Fitzwater, a spokeswoman for the West Virginia Office of Miners' Health Safety and Training.
Proper ventilation keeps explosive methane gas and coal dust at safe levels.
The investigative teams will include representatives of the state and federal mine safety agencies, Massey Energy and the United Mine Workers union. The union is the official representative of miners during the investigation.
In a written statement, Massey Energy said the beginning of this phase of the disaster investigation is "an important step to determine what happened here at Upper Big Branch."
One focus of the underground search for answers will be methane monitors and detectors that sound warnings and even shut down mining machines when methane approaches dangerous levels.
"The explosion was extremely massive," state mine safety director Ron Wooten said in an interview with West Virginia Public Broadcasting. "There had to have been a large accumulation of methane."
One mystery about the blast is why methane monitors didn't warn of high gas levels.
"There had to have been a reason that the methane monitors, if they were turned on, and the methane detectors, if they were turned on, didn't give some kind of warning," he said.
Disabling monitors without a legitimate reason is a crime. Wooten also suggested that a small and isolated methane leak may have occurred away from the monitors.
"There are a lot of things that could have happened," Wooten told WVPB. "What we can't understand is how it was so catastrophic, so instantaneous as to basically stop 29 people in their tracks."