"We intend to co-operate in all phases of the accident investigation."

'Big lie'

Earlier this week, Massey accused unions of spreading a "big lie" that the company had traded safety for profit, and Bobby Inman, the company's director, dismissed calls by shareholders to remove Don Blankenship, the chief executive officer.

Inman said Blankenship had borne the brunt of criticism for the accident at the mine and said firing the CEO was "not in the cards."

Last week, Blankenship denied the explosion was the result of "wilful disregard" for safety regulations but acknowledged that eight Massey mines were targets for an inspection "blitz" by the Mine Safety and Health Administration, the agency that regulates the industry.

Organised labour groups have said that Massey's safety record is inferior to that of most other major coal companies.

"The big truth is that 52 people have been killed on Massey property since 2000," Phil Smith, a spokesman for the United Mine Workers of America told Reuters.

"No other coal company has had even half that."