Rescue teams were still holding out hope for four workers missing in a coal mine where a massive explosion killed 25, but officials said Tuesday that the chances were slim that the miners survived.

The suspended rescue mission would resume after bore holes could be drilled to allow for toxic gases to be ventilated from Massey Energy Co.'s sprawling Upper Big Branch mine about 30 miles (50 kilometers) south of Charleston, state and federal safety officials said.

"All we have left is hope, and we're going to continue to do what we can," Kevin Stricklin, an administrator for the federal Mine Safety and Health Administration, said at a news conference. "But I'm just trying to be honest with everybody and say that the situation does look dire."

Though the cause of the blast was not known, the operation run by Massey subsidiary Performance Coal Co. has a history of violations for not properly ventilating highly combustible methane gas, safety officials said.

Stricklin said officials had hoped some of the missing survived the initial blast Monday afternoon and were able to reach airtight chambers stocked with food, water and enough oxygen for them to live for four days. However, rescue teams checked one of two nearby and it was empty. The buildup of toxic methane gas _ a constant problem at the mine _ and of carbon monoxide prevented teams from reaching other chambers, officials said.

A total of 29 miners were in the area during a shift change when the blast happened, Stricklin said. Some may have died in the blast and others when they breathed in the gas-filled air, he said. Seven bodies have been recovered and identified, but the other 18 have not, said Gov. Joe Manchin, who returned to the state after getting the news.

"Everybody's just heartbroken over this and the impact on these families," said mine safety director Joe Main, who planned to go to West Virginia.

State mining director Ron Wooten said though the situation does not seem promising, rescuers weren't done.

"We haven't given up hope at all," he said.

It is the most people killed in a U.S. mine since 1984, when 27 died in a fire at Emery Mining Corp.'s mine in Orangeville, Utah. If the four missing bring the total to 29, it would be the most killed in a U.S. mine since a 1970 explosion killed 38 at Finley Coal Co., in Hyden, Kentucky.

After a record low 34 deaths last year, Main said he and others believed coal mining had turned the corner on preventing fatal accidents.

"There's always danger. There's so many ways you can get hurt, or your life taken," said Gary Williams, a miner and pastor of a church near the southern West Virginia mine. "It's not something you dread every day, but there's always that danger. But for this area, it's the only way you're going to make a living."

Benny R. Willingham, 62, who was five weeks away from retiring, was among those who perished, said his sister-in-law Sheila Prillaman.

He had mined for 30 years, the last 17 with Massey, and planned to take his wife on a cruise to the Virgin Islands next month, she said.

Prillaman said family members were angry because they learned of Willingham's death after reading it on a list Massey posted, instead of being contacted by the company, which said it wouldn't release names until next of kin were notified.

"The families want closure," Manchin said at a news conference. "They want names ... these families are good people. Hard-working people. They understand the challenges. Right now I told them to do what they do best. Love each other and come together as a family."

Manchin said the explosion was massive and that the situation looks bleak, but that miracles can happen and pointed to the 2006 Sago Mine explosion that killed 12.

Crews found miner Randal McCloy Jr. alive after he was trapped for more than 40 hours in an atmosphere poisoned with carbon monoxide.

Nine miners were leaving on a vehicle that takes them in and out of the mine's long shaft when a crew ahead of them felt a blast of air and went back to investigate, Stricklin said.

They found nine workers, seven of whom were dead. Others were hurt or missing about a mile and a half inside the mine, though there was some confusion over how many.

Massey Energy, a publicly traded company based in Richmond, Virginia, has 2.2 billion tons of coal reserves in southern West Virginia, eastern Kentucky, southwest Virginia and Tennessee, according to the company's Web site. It ranks among the nation's top five coal producers and is among the industry's most profitable. It has a spotty safety record.

In the past year, federal inspectors fined the company more than $382,000 for repeated serious violations involving its ventilation plan and equipment at Upper Big Branch. The violations also cover failing to follow the plan, allowing combustible coal dust to pile up, and having improper firefighting equipment.

Upper Big Branch has had three other fatalities in the last dozen years.

Methane is one of the great dangers of coal mining, and federal records say the Eagle coal seam releases up to 2 million cubic feet (60,000 cubic meters) of methane gas into the Upper Big Branch mine every 24 hours, which is a large amount, said Dennis O'Dell, health and safety director for the United Mine Workers labor union.

In mines, giant fans are used to keep the colorless, odorless gas concentrations below certain levels. If concentrations are allowed to build up, the gas can explode with a spark roughly similar to the static charge created by walking across a carpet in winter, as at the Sago mine.

Since then, federal and state regulators have required mine operators to store extra oxygen supplies. Upper Big Branch uses containers that can generate about an hour of breathable air, and all miners carry a container on their belts besides the stockpiles inside the mine.

The mine has 19 openings and roughly 7-foot ceilings. Inside, it's crisscrossed with railroad tracks used for hauling people and equipment. It is located in a mine-laced swath of Raleigh and Boone counties that is the heart of West Virginia's coal country.

The seam produced 1.2 million tons of coal in 2009, according to the mine safety agency, and has about 200 employees, most of whom work underground on different shifts.

"The federal Mine Safety and Health Administration will investigate this tragedy, and take action," U.S. Secretary of Labor Hilda L. Solis said in a statement. "Miners should never have to sacrifice their lives for their livelihood."

___

Associated Press writers Allen G. Breed, Vicki Smith and Tim Huber in West Virginia and Sam Hananel in Washington contributed to this report.