MONTCOAL, W.Va.  - Six miners were killed and at least 21 unaccounted for Monday in an explosion at an underground coal mine, the state mining director said. Ron Wooten said the blast was reported around 3 p.m. at Massey Energy's Upper Big Branch mine in Raleigh County, about 30 miles south of Charleston.

The company did not provide details on the extent of the damage. A Boone County ambulance dispatcher also said he has the same number of fatalities and missing miners. The mine is operated by Massey subsidiary Performance Coal Co. Mine emergency crews from the State Office of Miners' Health Safety and Training were headed to the scene, but agency spokeswoman Jama Jarrett had no more details.

At least three fatalities have happened at the mine in the past dozen years. In 1998, a worker was killed when a support beam collapsed, dumping bags of cement mix and other materials onto the man, according to a report from the federal Mine Safety & Health Administration. Federal investigators blamed poor welding and construction.

In 2001, another worker at the mine died after a portion of roof fell in on him, and an electrician died after being electrocuted while repairing a shuttle car there in 2003. The mine produced 1.2 million tons of coal in 2009, according to the Mine Safety and Health Administration. Massey Energy is a publicly traded company based in Richmond, Va., that has 2.2 billion tons of coal reserves in southern West Virginia, eastern Kentucky, southwest Virginia and Tennessee, according to the company's Web site.

In 2006, 12 miners died in a methane explosion at the Sago Mine in West Virginia. Six were killed in the collapse of the Crandall Canyon mine in Utah in 2007.