MONTCOAL, W.Va. -- Rescue teams were rushing back into Massey Energy's Big Branch Mine in Raleigh County early this afternoon in a fourth desperate attempt to find four miners who remain unaccounted for following a horrific explosion four days ago in Raleigh County.Two eight-person teams were dispatched deep into the mine, where they hoped to find that the four miners had made it to an airtight rescue chamber that could have allowed them to survive since the disaster 96 hours ago."They are gearing up and going underground basically as we speak," Kevin Stricklin, coal administrator for the U.S. Mine Safety and Health Administration, said during a 2:30 p.m. briefing.MSHA, state officials and Massey decided to send rescue teams back into the mine after it appeared that nitrogen being pumped into the tunnels was pushing high levels of carbon monoxide back out of the sprawling mine.Stricklin said it could take 3 to 4 1/2 hours for the rescuers to make it to the area where they believe the miners could be.During the briefing, Stricklin also delivered bad news that a borehole that was drilled into the mine near the rescue chamber hit a solid block of coal -- making it useless for inserting a camera to try to locate the miners if they are in that area."We can put a camera down in it, but there will be nothing to see," Stricklin said.Earlier today, rescue teams were forced to retreat again out of the mine after they encountered a large amount of smoke from a fire near the mine's longwall section.That development was another major setback, and efforts to rescue the miners still unaccounted for grew more desperate as the rescue operation in Southern West Virginia neared the end of its fourth full day."Not a whole lot has really gone our way," Stricklin said.During an earlier mission into the mine that started at about 12:45 a.m. today, two specially trained and equipped rescue teams found one refuge chamber in the mine that had not been used, but could not reach a second chamber. They will not be able to get to the second refuge chamber within 96 hours of Monday's explosion -- the amount of time the chambers are supposed to keep people alive. That would be about 3 p.m. today.Technically, the 96-hour limit is based on having at least 15 people inside the refuge chamber, so it's possible fewer people could survive longer."The big decision, the very tough decision, was to pull the rescue team out and not put them in harm's way based on the smoke we saw," Stricklin said early in the day.The massive explosion deep inside the mine has already claimed 25 lives, making it the worst U.S. coal mining disaster since 1984. Two more miners were injured and were taken to local hospitals. Stricklin said that the search is still a rescue operation and that will not change until they learn if the last refuge chamber has been deployed."The bottom line is this, we told [the families] that the first chamber was checked along the long wall and it was not deployed," Manchin said. "They've got one opportunity, we've all got one opportunity, a sliver of hope, a miracle if you will, that the other chamber has been deployed."Ron Wooten, director of the West Virginia Office of Miners Health, Safety and Training, said, "We have hope. We still have hope."Officials were being especially careful, though, because of the deaths of mine rescue crew members in a secondary mine explosion in Kentucky in 1976 and in a follow-up pillar collapse, or burst, just three years ago at the Crandall Canyon Mine in Utah."We have to do what we think is right," Stricklin said. "We have to take away any possibility of another explosion."After the rescue teams were pulled out this morning, workers began to pump nitrogen into the mine. The nitrogen reduces the oxygen level, which should make the air less combustible and put out the fire.When asked why nitrogen wasn't put in earlier, Stricklin said it was, yet again, a change of plans."The plan was to put nitrogen in if [gases in the mine] came close to the explosive range. What we didn't expect was there to be smoke from a fire, and that changed what we had to do," he said.In an e-mail message, though, a Department of Labor spokesman said that Massey had "dragged its feet" in having the nitrogen delivered to the mine site."We asked the company for it two days ago," said department spokesman Carl Fillichio. "We had to keep asking for it."Earlier Thursday, rescue teams were pulled from the mine after a three-hour mission that got them within 1,000 feet of a refuge chamber where they hoped the miners took shelter. But the rescuers were called back from the mine Thursday morning after repeated sampling showed unsafe air quality that could cause another explosion.Stricklin said Thursday the explosion filled the mine with an "explosive mixture" of high levels of carbon monoxide and methane, along with low levels of oxygen."It tells us it was a very violent explosion," he said.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.Any chance of survival the miners had hinged on using self-contained self-rescuers, or SCSRs, to make their way to refuge chambers stocked with enough air, food and water to last four days.An SCSR is a breathing apparatus that generates about an hour's worth of oxygen that each miner carries. 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 a search 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. However, 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.