MONTCOAL, W.Va. - Two full days after the worst U.S. mining disaster in a generation, dangerous gases underground prevented rescuers late Wednesday from venturing into the Upper Big Branch coal mine to search for any survivors of the explosion that killed at least 25 workers.

Crews drilled holes deep into the ground to release the gases. By evening, a federal safety official said the levels of lethal carbon monoxide and highly explosive hydrogen and methane measured at the top of the holes were steadily dropping. Officials by late evening planned to test levels at the bottom of the holes to determine if three teams of five rescuers each can enter.

"We just can't take any chances" with the lives of rescuers, Kevin Stricklin of the federal Mine Safety and Health Administration had said earlier. "If we're going to send a rescue team, we have to say it's safe for them to go in there."

Officials could not say specifically when rescuers might be able to go in, but if the readings at the bottom were good, they want them on the move as soon as possible, Stricklin said.

Stricklin said relatives of the miners backed the decision to hold off for now.

"We've asked the families to be patient," he said earlier in the day.

Gov. Joe Manchin and others saw only a "sliver of hope" that the miners survived by reaching one of the shaft's rescue chambers, which are stocked with food, water and enough oxygen to last four days. Workers planned to drill another hole so they could lower a camera into one of the airtight chambers to see if anyone managed to get inside.

"We've been working against long odds from day one," Manchin warned.

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

The mine's owner, Massey Energy Co., has been repeatedly cited for problems with the system that vents methane and for allowing combustible dust to build up. On the day of the blast, MSHA cited the mine with two safety violations - one involving inadequate maps of escape routes, the other concerning an improper splice of electrical cable. However, Stricklin said the violations had nothing to do with the blast.

Massey CEO Don Blankenship has strongly defended the company's record and disputed accusations from miners that he puts coal profits ahead of safety.

As of late Wednesday, there had been no signs of life deep underground since the explosion. During the drilling of the ventilation holes, rescuers banged on a pipe for about 15 minutes but got no response. Miners are trained to bang on drilling equipment and ceiling bolts if trapped.

Family members could do little but wait.

Alice Peters said she was told her 47-year-old son-in-law, Dean Jones, was among the missing, though Massey said it does not know which four miners might be alive.

Peters said Jones' wife, Gina, has been at the mine site since the explosion and would not leave. "She's not doing too good," Peters said. "They told them to go home because they weren't going to let the mine rescuers back in. They're still drilling."

Seven bodies were pulled out after the explosion, and two miners were hospitalized. Manchin said Wednesday that one was doing well and the other was in intensive care. Eighteen bodies remained in the mine, but emergency workers were able to identify only four before methane forced them out Monday.

During the drilling of the ventilation holes, the amount of methane, hydrogen and carbon monoxide coming out of the mine was so high - the carbon monoxide was 280 times above safe levels - that ventilation had to be set up at the surface to protect the rescue workers, Stricklin said.

"It was to the point that it was affecting the drillers," Stricklin said. "They were standing right next to where noxious gas was coming out."

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.

Doug Griffith, another of William Griffith's brothers and also a miner, sat down with his family after getting a briefing on the rescue effort, said his wife, Cindi.

"He just said we really need to prepare for the worst," she said. "They don't feel like there's any hope."

The quality and quantity of coal produced at Upper Big Branch make the mine one of the gems of Massey's operation. The mine produced more than 1.2 million tons of coal last year and uses the lowest-cost underground mining method, making it more profitable. The mine 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 family of 50-year-old Ricky Workman was told he was among those missing, said a niece, Tammy Cruz of Cleveland. Cruz said Workman had complained to family members about ventilation problems in the mine.

"He'd be complaining for weeks," Cruz said. "And he had told them, 'Does somebody else have to die before you do something about this?' He knew this was coming."

Workman's family waited with other families in seclusion at the mine complex.

"Ricky's got a young soul," Cruz said. "He's a fighter. He's a smart guy. Hopefully he got to one of those safe places and they're going to pull him out alive."

Associated Press writers Greg Bluestein, Allen G. Breed, Vicki Smith, Tom Breen, 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.