High levels of dangerous methane gas made it impossible for rescuers to venture inside a coal mine Wednesday to search for survivors of an explosion that killed 25 workers.

Crews drilled holes to release the gas, but by late Wednesday afternoon the levels were still far too high for searchers to safely enter the Upper Big Branch mine to look for four people still missing in the worst U.S. mining accident in more than two decades.

Workers planned to drill another hole so they could lower a camera into a rescue chamber to see if anyone was inside, Kevin Stricklin of the Mine Safety and Health Administration said at a briefing Wednesday. He said relatives of those in the mine backed the decision to hold off on sending in rescue crews for now.

"We've asked the families to be patient," Stricklin said.

The disaster has brought new scrutiny for Massey, which has been repeatedly cited for problems with the system that ventilates explosive methane gas and for allowing combustible dust to build up. The federal Mine Safety and Health Administration on Wednesday appointed a special team of investigators to look into the blast, which officials said may have been caused by a buildup of methane.

Like many other mine operators, Massey frequently sidesteps hefty fines by aggressively appealing safety violations at the mine, according to an Associated Press analysis of mine safety records.

Rescuers hoped the four miners might somehow have reached a chamber where they could survive for four days, though they acknowledged the odds were against them. Rescuers banged on a drill pipe for about 15 minutes after the first hole was complete but got no response.

Family members could do little but wait.

"We've been working against long odds from day one," Gov. Joe Manchin said at a briefing Wednesday afternoon.

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

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 only able to identify four before methane forced them out Monday.

Peters said Jones' wife, Gina, has been at the mine site since the explosion and will 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."

Crews later planned to set off small explosions on the surface to send a seismic signal to the mine. Miners are trained to bang back on the drill's casing, and sections of mine roof contain numerous metal bolts that trapped miners can bang on to signal their presence.

Once rescuers can get into the mine, it could take less than two hours for a team to get far enough inside to check for survivors, depending on conditions, Stricklin said. They would be about 1,000 feet below the surface, and at least a mile-and-a-half from the entrance.

The quality and quantity of coal produced at Upper Big Branch make the mine one of 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.

Congressman Nick Rahall, a Democrat whose district includes the mine, said Wednesday that at least three Upper Big Branch miners had come to him since the explosion to say they were concerned about methane levels.

"The feelings were that they were not walking into a safe working environment," he said. They asked that their identities be kept secret because they feared losing their jobs.

Federal regulators probing the explosion plan to review Massey's safety violations, many of which involved venting methane gas. If the odorless, colorless gas is not kept at safe levels, a small spark can ignite it.

Massey is still contesting more than a third of all its violations at the Montcoal, W.Va., mine since 2007. In the past year, federal inspectors have proposed more than $1 million in fines for violations at the mine. Only 16 percent have been paid.

Bombarding federal regulators with appeals is an increasingly common industry tactic since the 2006 Sago mine disaster that killed 12 led to stiffer fines and new enforcement to punish the worst offenders, according to AP's review of records from the Mine Safety and Health Administration.

In an interview Tuesday with AP, Massey CEO Don Blankenship downplayed the link between the ventilation system and the accident.

"I don't know that MSHA thought there was a problem," he said.

He said the families of those killed were angry and made "a lot of derogatory comments" during meetings with company officials. He said he did not directly address them.

"They're looking for some way to release their anger and that's just the way it is," he said.

He said the chances of miners making it out of the violent explosion alive were dim, and "I think it dims every day."

The death toll in Monday's explosion was the highest in a U.S. mine since 1984, when 27 people 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 to die in a U.S. coal mine since a 1970 explosion killed 38 at Finley Coal Co. in Hyden, Ky.

Larry Asbury, whose son is on a mine rescue team, joined about 50 mourners who packed the creaky pews of the modest St. Joseph Catholic Church a few miles from the disaster Tuesday to honor the victims and pray that the missing turn up safe.

"The coal community is coming together and praying for miners and their families," he said. "It's just so important to show the community this kind of support."

Manchin said the first drill hole entered the section of the mine about a football field's length away from a rescue chamber where officials hope the miners sought refuge.

Searchers would have to navigate in the darkness around debris from structures shattered by the explosion and around sections of track that were "wrapped like a pretzel," said Stricklin, the federal mine administrator.

"There's so much dirt and dust and everything is so dark that it's very easy, as hard as it may seem to any of us outside in this room, to walk by a body," Stricklin said.

Though the situation looked bleak, the governor 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.

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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, and Sam Hananel in Washington contributed to this report.