MONTCOAL, W.Va. - Anxious search crews prepared late Thursday to pump nitrogen into a coal mine where an explosion killed 25, a last-ditch bid to flush dangerous gases out and allow rescuers to reach up to four survivors.

Safety officials conceded late Thursday that any hope of finding workers alive in rescue chambers three days after the seismic blast was quickly fading.

"We committed to the families we were going to get into the chambers within 96 hours and we're doing everything in our power to do that," said Kevin Stricklin, a coal administrator from the federal Mine Safety and Health Administration.

Air holes drilled into the Upper Big Branch mine have ventilated noxious and explosive gases but not enough to make the atmosphere safe. They wanted to test the air one more time before starting to pump in nitrogen to neutralize the air enough for crews to enter.

At a 10:30 p.m. briefing, Stricklin said officials wanted to continue testing another hour to be sure dangerous gases continued to dissipate. If not, trucks with nitrogen were available to start pumping.

"We're just moving as quickly as we can," Gov. Joe Manchin said. "We want to bring the loved ones back."

The teams expected to be able to descend sometime early today, if the final sample showed signs of improving air quality.

Stricklin estimated it should take the teams an hour and a half to reach the search area, half the time of a morning attempt.

Earlier in the day, searchers came within 500 feet of a rescue chamber where possible survivors may have taken refuge, but were told to abandon their mission because the explosive mix of gases had become too dangerous.

Teams spent more than four hours in the morning working their way by rail car and on foot through the mine. When told to abandon their mission, they were angry, but their safety was paramount, said Chris Adkins, chief operating officer for mine owner Massey Energy Co.

Rescue teams were headed first to an airtight chamber that has at least four days worth of food, water and oxygen.

During the morning foray, rescue crews did not get far enough to see the bodies of the dead or if anyone had made it to the chamber. They knew where the bodies would be because rescuers made it that far before gases forced them out of the mine after the explosion Monday.

Massey's chief executive officer, Don Blankenship, continued to defend his company's record and disputed accusations from miners that he puts coal profits ahead of safety.

Throughout communities surrounding the mine, families and neighbors anxiously watched the slow progress.