(AP) — Rescuers running on adrenaline waited Thursday for a massive drill to vent noxious gas so they could safely resume the underground search for four coal miners missing since an explosion that killed 25 colleagues.

They had spent more than four hours working their way through the Upper Big Branch mine by rail car and on foot, but had to turn back because of an explosive mix of gases in the area they needed to search.

Crews at the surface resumed drilling started earlier in an effort to get fresh air into the mine and hoped 32 rescuers could return by about 7 p.m.

They had made it within 500 feet of an airtight chamber with four days worth of food, water and oxygen where they hoped the miners might have sought refuge after the worst U.S. mining disaster in more than two decades.

Chris Adkins, chief operating officer for mine owner Massey Energy Co., said the rescue teams were "very angry" when told to abandon the mission, but their safety was paramount. He said the teams are off their feet and resting, but too anxious to sleep.

Despite the increasingly slim chance of finding anyone alive, Adkins said he considers the effort a rescue mission.

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"I still believe in God, I believe, and I'm not gonna give up," he said.

The 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 after the explosion Monday before gases also forced them out of the mine.

Officials were not sure what caused the high gas levels this time but said a drop in barometric pressure as a storm rolled in might be to blame.

The rescue crews were leaving their equipment behind so they did not have to lug it back in with them when they returned.

Gov. Joe Manchin told families waiting at the mine complex that the next several hours would be a good time to take a break and get some sleep or a shower.

Kevin Stricklin of the federal Mine Health and Safety Administration said the families understood the need to pull rescuers out.

"It's a roller coaster for these people," Stricklin said. "It's very emotional. You can only imagine what it would be like."

Rescuers had already had to wait to enter the mine until crews drilled holes deep into the earth to ventilate lethal carbon monoxide and highly explosive hydrogen as well as methane gas, which has been blamed for the explosion. The air quality was deemed safe enough early in the day for four teams of eight members each to go in, but later tests showed the air was too dangerous to continue.

Adkins said rescue teams described seeing evidence of "a horrendous explosion and a lot of destruction."

He also said they may have found an alternate route that will allow them to get where they need to be faster when they can safely go back in.

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Once that happens, rescuers will have to walk through an area officials have described as strewn with bodies, twisted railroad track, shattered concrete block walls and vast amounts of dust. Each team member wears 30 pounds of breathing equipment, lugs first-aid equipment and must try to see through total darkness with only a cap lamp to light the way.

Officials and townsfolk and even some family members acknowledged they didn't expect to find any of the four missing miners alive more than two days after the massive explosion.

"In my honest opinion, if anyone else survives it, I will be surprised," said James Griffith, who works at the mine. His brother, William "Bob" Griffith, went to work Monday and never came home. William Griffith's brother-in-law, Carl Acord, died in the explosion.

Two miners were injured in the blast but managed to get out. One was in intensive care and the other was in good condition, but Manchin described him as being "in total withdrawal" and said he does not want to talk to anyone.

Seven bodies were pulled out Monday and at least 18 remained in the mine.

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The federal mine agency has appointed a team of investigators to look into the blast, which officials said may have been caused by a buildup of methane.

The mine is outfitted with air-quality sensors that shut down some of the mining machinery when methane levels reach a high level.

Manchin said it's unclear whether the methane levels reached that point prior to the blast, or whether the sensors detected it. However, the positioning of several bodies in an underground rail car begs the question: "Did a sensor not go off?" the governor said Thursday.

"The miners that they found, it doesn't look like anyone was alarmed or warned that something as this horrific was going to happen," Manchin said. "When you find people just sitting in the mantrip, as if they're just waiting to go out and they're still there? That tells me there was no panic."

Even with high gas levels inside the mine, there must have been a source of ignition, Manchin said, and it's unclear what that might have been.

Massey has been repeatedly cited for problems with the system that vents methane and for allowing combustible dust to build up, including two large fines assessed in January when federal inspectors found dirty air flowing into an escapeway where fresh air should be, and an emergency air system flowing in the wrong direction. Miners were so concerned about the conditions that several told their congressman they were afraid to go back into the mine.

Even the day of the blast, the federal mine agency cited the mine for two safety violations, one involving inadequate maps of escape routes, the other concerning an improper splice of electrical cable. Stricklin said, however, that those violations had nothing to do with the explosion.

Massey CEO Don Blankenship has strongly defended the company's record and disputed accusations from miners that he puts coal profits ahead of safety. On Thursday, he began using the social networking site Twitter to communicate about the disaster.

"Pray for the families and the rescue workers," he tweeted. He also praised the rescue efforts and got in a dig at what he called the "indignity of much of the media."

The Upper Big Branch mine produced more than 1.2 million tons of coal last year and uses the lowest-cost underground mining method, making it more profitable. It produces metallurgical coal that is used to make steel and sells for up to $200 a ton — more than double the price for the type of coal used by power plants.

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Associated Press Writers Allen G. Breed, Greg Bluestein, Tom Breen, Dena Potter, Tim Huber and John Raby and videojournalist Mark Carlson in West Virginia; Mitch Weiss and Mike Baker in North Carolina; Ray Henry in Atlanta; and Sam Hananel in Washington contributed to this report.