“All we know now is, this is an awful disaster,” Representative Nick J. Rahall II said as he arrived at the mine site, which is in his district. “This is the second major disaster at a Massey site in recent years, and something needs to be done.”

In a statement, Massey’s chief executive officer, Don Blankenship said mine rescue teams and state and federal officials were responding to the explosion. “We want to assure the families of all the miners we are taking every action possible to locate and rescue those still missing,” Mr. Blankenship said.

Phil Smith, a spokesman for the United Mine Workers of America, said that the mine was nonunion but that the union had dispatched a team to advise on the rescue and to help the families of the trapped or dead miners.

Michael Mayhorn, an emergency dispatcher for Boone County, said that at least 20 ambulances and three helicopters had been dispatched from nearby counties, and that the state medical examiner was heading to the scene. At least one miner was evacuated by helicopter, he said.

Image Diana Mccoure waited for news from Upper Big Branch Mine-South at the New Life Assembly in Whitesville, W. Va. Credit... Andrew Spear for The New York Times

Dennis O’Dell, an official with the union who was in contact with state and federal safety officials, said the current theory was that the explosion might have been caused by a buildup of methane gas in a sealed-off section of the mine. A similar type of explosion occurred in the 2006 Sago mining disaster, which left 12 miners dead after trapping them underground for nearly two days.

Mr. O’Dell said some officials believed the ignition source for the explosion on Monday might have been a device that carries mine personnel to and from the work area.