And West Virginia requires all underground mines to have wireless communications and tracking systems designed to survive explosions and other disasters.

But Massey did not provide details on the extent of the damage or if other miners had made it out on their own.

Spotty safety record

Federal records show three miners have died on the job at Upper Big Branch mine, operated by Massey subsidiary Performance Coal, since 1998.

Massey ranks among the top five coal producers in the US and is among the industry's most profitable, but it has a spotty safety record.

The federal mine safety administration fined Massey a then-record $1.5m for 25 violations that inspectors concluded contributed to the deaths of two miners trapped in a fire in January 2006.

The company later settled a lawsuit naming it, several subsidiaries and Blankenship as defendants.

Subsidiary Aracoma Coal later paid $2.5m in fines after the company pleaded guilty to 10 criminal charges in the fire.

Last year, the number of miners killed on the job in the US fell for a second straight year to 34, the fewest since officials began keeping records nearly a century ago.

That was down from the previous low of 52 in 2008.