Four eight-man teams initially entered the Raleigh County operation about 5am local time Thursday morning, and state governor Joe Manchin said in a 10.20am briefing that they had made it to the proximity of the Longwall 22 section where the workers were believed to be, and within 1000 feet of the refuge chamber there.

However, as of that briefing rescuers were being pulled out of the mine and were en route to the surface, according to US Mine Safety and Health Administration coal administrator Kevin Stricklin.

“We’ve had a setback,” he announced, noting that “consecutive air samples indicated an explosive mixture” once again in the area, leading to a decision to remove the rescue teams.

“We think they are in danger. That is the whole intent of pulling them [the 32 rescuers] out,” Stricklin added, quickly pointing out that the effort was still considered a rescue mission.

Once crews are safely out of the mine, drilling will continue on the second borehole, which was about 70ft from its destination about 1100ft underground as of late Thursday morning. The hole, measuring about 6 inches in diameter, will double ventilation of the toxic gas build-up underground.

Work will also continue on the remaining three boreholes, including one that is being aimed near the refuge chamber where officials believe the miners could be located.

Stricklin said that air samples were being taken every 15 minutes and the rescuers would be permitted to re-enter and continue their work once acceptable levels were again reached. Authorities hope that will occur “within the next few hours” and crews have left their supplies underground this morning so the materials do not have to be carried into the section again.

While the cause of the accident has not been officially confirmed, many experts have speculated to the media that it may have been caused by an explosion of methane gas, potentially fueled by coal dust.

Officials have said since Monday evening that any chance the four miners have survived hinges upon their use of self-contained self-rescuers and the refuge chambers located throughout the mine.

It was discovered overnight that three SCSRs had been removed from emergency stockpiles, leading rescuers to believe some workers could have survived the initial explosion, according to some news outlets.

"[They] know the odds are not in our favor," Manchin said of the families still waiting to hear of the fate of the final four.

“But that's what we're still holding on to."

Stricklin insisted officials and crews were remaining focused.

"It is what it is, and this is what we're dealing with right now. It's nothing compared with what the families are going through … and I always try to keep that in mind that we'll do whatever it takes to bring this to some type of conclusion."

The 200-worker Upper Big Branch South mine, which has one longwall and five active miner sections, extracts from the Eagle seam. The mine is operated by Massey division Performance Coal.

Keep reading ILN for the latest information on the Upper Big Branch explosion as it becomes available.