MONTCOAL, W.Va. -- Rescuers continued a frantic race against time late Thursday night, hoping toxic gas levels dropped enough that rescuers could get back into a Raleigh County mine, but also plotting other efforts they hoped could save four miners still missing after a huge explosion four days ago.

"Everything is simultaneous," said Kevin Stricklin, coal administrator for the U.S. Mine Safety and Health Administration. "We've got four or five plans going on at the same time."

Toxic gas levels in the mine were trending downward, but were still too high to send rescuers in as of about 11 p.m. If they continued to drop, though, rescue coordinators hoped to send specially trained and equipped rescue teams back in as early as midnight to resume their search.

If gas levels don't come down enough, or if they go back up, crews plan to pump nitrogen into the mine to make the atmosphere inert. Also, they are continuing plans to lower a camera into a borehole to look for the miners.

Stricklin said officials were intent on getting rescue teams back in the mine late Thursday night or very early this morning, in the hopes of reaching the area where the four missing miners are believed to be within 96 hours of the explosion, which occurred at about 3 p.m. Monday. That mark -- the limit of water, breathable air and other supplies in airtight rescue chambers in the mine -- would be mid-afternoon today.

"We have to get people in there tonight," Stricklin said. "We committed to the families that we would get into the chambers within 96 hours and we're going to do everything in our power to do that."

Technically, if the miners made it to a rescue chamber, they would be able to survive longer than four days. The structures were stocked for 15 miners to survive four days, so four miners could survive longer.

The massive explosion deep inside Massey Energy's Upper Big Branch Mine has already claimed 25 lives, making it the worst U.S. coal mining disaster since 1984. Two more miners were injured, but one of them was released Thursday from a local hospital. The other remains in intensive care, but more information on his condition has not been made public.

Earlier Thursday, rescue teams were pulled from the mine after a roughly three-hour mission got them within 1,000 feet of a refuge chamber where they hope the miners took shelter. But the rescuers were called back from the mine this morning after repeated sampling showed unsafe air quality that could threaten another explosion.

Stricklin called that development a setback, while Gov. Joe Manchin said it was "the worst scenario" for rescuers hoping for a miracle.

Stricklin said the explosion has filled the mine with an "explosive mixture" of high levels of carbon monoxide and methane, along with low levels of oxygen, and that officials would not risk rescuers' lives in the search for the missing miners.

"It tells us it was a very violent explosion," he said. "There is so much gas built up in the area that it's taking us awhile to ventilate."

Stricklin said changes in barometric pressure, prompted by a weather front moving through the Raleigh County area, also could be complicating the rescue. Barometric pressure changes increase the natural tendency of geology in underground coal mines to release explosive methane. When a cold front moves in and the pressure drops, more methane is released.

Rescue crews were sent into the mine shortly before 5 a.m. Thursday, after being pulled early two days before because of dangerous gases.

They again reached about the same point -- an area near where the mine's longwall section begins, near one of two rescue chambers that have not yet been checked for potential survivors. Rescuers believe that the last location of three of the miners was in a separate tunnel north of there, and that one other miner was last known to be somewhere on the longwall section.

Crews on the surface planned to immediately resume drilling additional boreholes to try to speed up the ventilation of the mine and sweep the explosive and toxic gases out so that rescuers can resume their work, officials said.

Mine safety officials repeatedly called Thursday morning's development a setback, but also insisted that the operation remained a rescue effort, not a project to recover the miners' bodies.

"We couldn't let the rescue teams underground any longer based on the readings," Stricklin said. "But nothing has changed. It's still a rescue operation."

Officials have said Monday's blast was likely caused by an explosion of methane gas, and was possibly fueled by the presence of coal dust.

"Everything is simultaneous," said Kevin Stricklin, coal administrator for the U.S. Mine Safety and Health Administration. "We've got four or five plans going on at the same time."

Toxic gas levels in the mine were trending downward, but were still too high to send rescuers in as of about 11 p.m. If they continued to drop, though, rescue coordinators hoped to send specially trained and equipped rescue teams back in as early as midnight to resume their search.

If gas levels don't come down enough, or if they go back up, crews plan to pump nitrogen into the mine to make the atmosphere inert. Also, they are continuing plans to lower a camera into a borehole to look for the miners.

Stricklin said officials were intent on getting rescue teams back in the mine late Thursday night or very early this morning, in the hopes of reaching the area where the four missing miners are believed to be within 96 hours of the explosion, which occurred at about 3 p.m. Monday. That mark -- the limit of water, breathable air and other supplies in airtight rescue chambers in the mine -- would be mid-afternoon today.

"We have to get people in there tonight," Stricklin said. "We committed to the families that we would get into the chambers within 96 hours and we're going to do everything in our power to do that."

Technically, if the miners made it to a rescue chamber, they would be able to survive longer than four days. The structures were stocked for 15 miners to survive four days, so four miners could survive longer.

The massive explosion deep inside Massey Energy's Upper Big Branch Mine has already claimed 25 lives, making it the worst U.S. coal mining disaster since 1984. Two more miners were injured, but one of them was released Thursday from a local hospital. The other remains in intensive care, but more information on his condition has not been made public.

Earlier Thursday, rescue teams were pulled from the mine after a roughly three-hour mission got them within 1,000 feet of a refuge chamber where they hope the miners took shelter. But the rescuers were called back from the mine this morning after repeated sampling showed unsafe air quality that could threaten another explosion.

Stricklin called that development a setback, while Gov. Joe Manchin said it was "the worst scenario" for rescuers hoping for a miracle.

Stricklin said the explosion has filled the mine with an "explosive mixture" of high levels of carbon monoxide and methane, along with low levels of oxygen, and that officials would not risk rescuers' lives in the search for the missing miners.

"It tells us it was a very violent explosion," he said. "There is so much gas built up in the area that it's taking us awhile to ventilate."

Stricklin said changes in barometric pressure, prompted by a weather front moving through the Raleigh County area, also could be complicating the rescue. Barometric pressure changes increase the natural tendency of geology in underground coal mines to release explosive methane. When a cold front moves in and the pressure drops, more methane is released.

Rescue crews were sent into the mine shortly before 5 a.m. Thursday, after being pulled early two days before because of dangerous gases.

They again reached about the same point -- an area near where the mine's longwall section begins, near one of two rescue chambers that have not yet been checked for potential survivors. Rescuers believe that the last location of three of the miners was in a separate tunnel north of there, and that one other miner was last known to be somewhere on the longwall section.

Crews on the surface planned to immediately resume drilling additional boreholes to try to speed up the ventilation of the mine and sweep the explosive and toxic gases out so that rescuers can resume their work, officials said.

Mine safety officials repeatedly called Thursday morning's development a setback, but also insisted that the operation remained a rescue effort, not a project to recover the miners' bodies.

"We couldn't let the rescue teams underground any longer based on the readings," Stricklin said. "But nothing has changed. It's still a rescue operation."

Officials have said Monday's blast was likely caused by an explosion of methane gas, and was possibly fueled by the presence of coal dust.

Complicating an already difficult situation for miners' families, rescuers know that there are 14 bodies of deceased miners still inside the mine, and four other miners whose whereabouts are unknown. But, rescuers did not identify the 14 bodies -- so, Manchin said, there are really 18 families who don't know for sure if their loved ones are alive or dead.

"This is the unknown fact, and it really adds a lot of anxiety," the governor said.

Any chance of survival hinged on the miners using self-contained self-rescuers, or SCSRs, to make their way to refuge chambers stocked with enough air, food and water to last for four days.

An SCSR is a breathing apparatus that generates about an hour's worth of oxygen. In the event of a catastrophe, miners are trained to first try to get out of the mine. If they can't, the next step is to make their way to a rescue chamber and hunker down.

Rescuers made an initial foray into the mine Monday night, but by early Tuesday were forced to pull out when they detected toxic and highly explosive gas levels underground. During that time, searchers were able to check one refuge chamber, which was empty.

They did discover that three SCSRs had been removed from emergency stockpiles, leading rescuers to believe that at least some miners had survived the initial blast.

In the meantime, workers drilled one ventilation shaft down 1,100 feet into the mine, with two additional holes near completion. Using large fans, they used the borehole to pump gas out of the mine so that rescuers could go back in.

Early Thursday, safety officials said the mine was again safe for rescuers, and four eight-man teams renewed their search for survivors.

The drawn-out, agonizing vigil brought back memories of the Sago disaster, in which 12 miners died after a methane gas explosion in the Upshur County mine in January 2006.

In the wake of Sago, and the death of two miners in a fire at Massey Energy's Aracoma mine days later, Congress passed the MINER Act, which required mine operators to update and improve their communication and tracking equipment.

The legislation also required the installation of SCSRs and refuge chambers in mines.

Critics of the MINER Act contend that it places too much emphasis on surviving a catastrophe after it occurs, rather than preventing it.

Manchin has promised a full state investigation into Monday's explosion, while Rep. Nick Rahall, the West Virginia Democrat whose district includes the state's southern coalfields, said Congress will hold hearings as well.

While the cause of the blast remains unknown, Stricklin grimly conceded earlier this week that the tragedy was preventable.

"We know it wasn't operating safely," he said, "or we wouldn't have had an explosion."

Staff writer Andrew Clevenger contributed to this report.

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