A team of federal and state investigators on Tuesday entered the underground coal mine in southeastern Kentucky where five miners died in an explosion several days earlier.

Concerns about flammable methane and poisonous carbon monoxide gases had kept the investigators out of the Kentucky Darby Mine No. 1 until its ventilation system could be repaired and the gases were brought down to safe levels.

The mine had been cited 41 times in the past five years for not cleaning up coal dust and other combustible materials, including three times this month, according to statistics from the U.S. Mine Safety and Health Administration.

Major targets of the investigation will be levels of potentially explosive coal dust and sealing material used to prevent methane from leaking out of previously mined areas, said Chuck Wolfe, a spokesman for the state Environmental and Public Protection Cabinet.

Wolfe would not comment on whether the mine's history of coal dust violations is a major concern to the investigators.

Kentucky Darby officials have declined to comment since the explosion Saturday. A call to the company's mine office was not answered Tuesday.

Since Kentucky Darby LLC took over as operator of the mine in May 2001, there had been 10 injuries but no deaths until Saturday's blast.

Wolfe said part of the investigation will look at whether naturally occurring methane accumulating in previously mined areas leaked around the unconventional seals used in the Harlan County mine and into active areas.

Rescuers reported the seals, similar to plastic foam, did not withstand the blast. The U.S. Mine Safety and Health Administration said it would require the use of concrete block seals while the issue is under review.

Three of those killed in Saturday's blast — Paris Thomas Jr., 35; Roy Middleton, 35; and George Petra, 49 — survived the initial blast but died of carbon monoxide poisoning, according to preliminary tests. Amon Brock, 51, and Jimmy D. Lee, 33, died of blunt force and heat injuries.

The men who died from carbon monoxide poisoning were using the same air pack model as the Sago Mine disaster victims in West Virginia, even though the lone survivor had questioned the reliability of the devices about a month ago. Twelve miners died in the Jan. 2 disaster at the Sago Mine.

On Tuesday, the wife of the sole Sago survivor called for answers about the air packs.

"Report after report has shown the rescuers did not work like they are supposed to," Anna McCloy, wife of Randal McCloy Jr., said in a statement.

David Dye, acting administrator of the federal mine agency, said in a statement that the air pack of Kentucky's survivor had worked properly. But Jeff Ledford, brother of survivor Paul Ledford, repeated Monday that his brother told him his air pack worked for only five minutes.

Kentucky legislators passed a measure requiring mines to store breathing devices underground and to set up lifelines to help miners find their way out, but that law does not take effect until July.

A U.S. Senate panel last week approved a bill that would require miners to have at least two hours of oxygen available instead of one as required under the current policy. It also would require mine operators to store extra oxygen packs along escape routes.

Saturday's blast was the deadliest mining incident in the state since 1989, when 10 miners died in a western Kentucky explosion, state officials said.

On Tuesday, a water truck operator was killed when the vehicle went over an embankment and crashed at a mountaintop coal mine in Martin County in eastern Kentucky, said Martin York, spokesman for the Kentucky Cabinet for Environmental and Public Protection.

The national death toll from coal mining accidents this year, including Tuesday's fatality, is 32, up from 22 in 2005.