A federal official vowed an "extensive investigation" of the company which owns the West Virginia mine that exploded Monday,.
Joseph Main, an official with the Mine Safety and Health Administration, told CBS' "The Early Show" Wednesday his agency will look into the history of safety enforcement at the Upper Big Branch mine in Montcoal, W.Va., owned by Massey Energy.
The record will show, Main said, that there was "some pretty heavy enforcement."
As of Wednesday, officials said there were no signs of life from the four missing miners. Further hampering the search was the buildup of methane gas and carbon monoxide, making it too dangerous for anyone to enter the mine.
Saying it could take months to thoroughly probe the cause of the explosion, Main said, "It's very obvious something went seriously wrong at this mine."
"This is going to be a pretty extensive investigation," he said.
Mine Explosion: No Signs of Life from Missing 4
In the month leading up to the fatal blast inside the Upper Big Branch mine, inspectors repeatedly cited Massey for serious safety infractions. Seven separate times the coal operator was hit with "ventilation violations" for failing to develop and carry out plans to monitor and remove dust and combustible gas from the mine, reports CBS News correspondent Bob Orr.
On Tuesday, Don Blankenship, the chairman and CEO of Massey Energy, joined CBS News anchor Harry Smith for an interview.
Smith: It's pretty well known now in the last 24 hours your company has been cited numerous times for ventilation violations. Could it be that just that kind of violation may be at root of the explosion that took place here yesterday?
Blankenship: Well, I think it's possible that anything could be the source of the fire or the explosion, I should say, at this point. But we don't know.
Smith: There is a sense among some in this community that your company has not done enough to keep the mines as safe as they ought to be.
Blankenship: Well, I've done everything I know to do and we've worked hard with the federal agencies and the state agencies to make all our of mines safe and we run a-hundred-and-something coal mines and we typically have a safe or better performance than the rest of the industry - 18 of the last 20 years we've had a better safety performance than the rest of the industry.
More on the mine explosion:
W. Va. Coal Mine Blast: The Victims
List Of Recent Fatal U.S. Mine Disasters
In Coal Mines, Risk of Death Is Part of Life
Mines not Paying Fines a Familiar Story
Gov.: "No Excuse" for Mine Safety Flaws
Eerie Statement from Miner Killed in Blast
Mining Company Previously Fined for Safety
Mine Explosion Rescue Efforts
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