MONTCOAL, W.Va. — Dangerous gases forced rescue crews to abandon the search Thursday for four coal miners missing since an explosion killed 25 colleagues in the worst U.S. mining disaster in more than two decades.

Rescuers had been working their way through the Upper Big Branch mine by rail car and on foot early Thursday, but officials said they had to turn back because of an explosive mix of gases in the area they needed to search.

“We think they are in danger and that’s the whole intent of evacuating them from the mine,” said Kevin Stricklin of the Mine Health and Safety Administration. “We couldn’t let the rescue teams underground any longer based on the readings.”

The rescuers made it to within about 500 feet of an airtight chamber with four days worth of food, water and oxygen where they hoped the miners might have sought refuge. They did not make it far enough to see the bodies of the dead or determine if anyone had made it to the chamber.

More photos below.

Stricklin acknowledged the evacuation was a setback. An executive with mine owner Massey Energy Co. said later that he hoped crews could get back into the mine around 7 p.m., after crews finished drilling a hole was to allow fresh air in. Officials weren’t sure what caused the gas levels to rise but said it could have been a drop in barometric pressure as a storm rolled in.

The rescue crews were leaving their equipment behind so they did not have to lug it back in with them when they returned. They knew where the bodies would be because rescuers made it that far after the explosion Monday before gases forced them out of the mine.

Stricklin said the families of the dead and missing understood the need to pull rescuers out again.

“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 on what officials were still calling a rescue mission, but later tests showed the air was too dangerous to continue.

J. Christopher Adkins, chief operating officer of Massey Energy Co., said at a briefing Thursday afternoon that rescuers were angry they had to turn back. He also described the scene when they went in.

“They see a horrendous explosion and a lot of destruction,” he said.

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

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 acknowledged they didn’t expect to find any of the four missing miners alive more than two days after the massive explosion. Poisonous gases have filled the underground tunnels since Monday afternoon’s blast.

“This was a scenario that we didn’t want,” Gov. Joe Manchin said as he briefed reporters about the evacuations. Families of those still in the mine continued to arrive at a training center there to await word of their fate, and Manchin estimated that perhaps 100 have gathered so far.

“They understand that if we have any hope of survival and they’re in a rescue chamber, then they’re OK,” Manchin said. “That’s the sliver of hope we have.”

Seven bodies had been brought out Monday and authorities hoped to recover 18 others known dead from the mine owned by Massey Energy Co., which has been cited for numerous safety violations.

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.

The Mine Safety and Health Administration has appointed a team of investigators to look into the blast, which officials said may have been caused by a buildup of methane.

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.

The confirmed death toll of 25 was the highest in a U.S. mine since 1984, when 27 people died in a fire at a mine in Orangeville, Utah. If the four missing bring the total to 29, it will be the worst U.S. coal mining disaster since a 1970 explosion killed 38 in Hyden, Ky.

The effect of so many sudden deaths in the area’s small coal-reliant communities started showing with obituaries for the victims appearing in local newspapers. The first five funerals were scheduled for Friday and Saturday.

Miner William “Bob” Griffith’s family was preparing for the worst. Griffith went to work Monday and never came home, said his brother, James Griffith, who also works at the mine. William Griffith’s brother-in-law, Carl Acord, died in the explosion.

“In my honest opinion, if anyone else survives it, I will be surprised,” James Griffith said.