(CNN) -- Rescuers were working Monday to free 12 miners trapped more than a mile underground after a coal mine explosion in West Virginia, officials said.
The blast was reported shortly before 8 a.m. ET, about eight miles south of Buckhannon near Tallmansville in Upshur County, said Louise Bleith, operations supervisor at the county's 911 center.
"Six came out," Bleith said. None of those who exited the mine required medical treatment, she said. (Watch what's being done to help trapped miners -- 1:28)
Twelve were still inside, said Steve Milligan, deputy director of the county's office of emergency management.
"Four miners went back in and they came upon a wall of debris, and they have not been able to contact the miners," he said. A rescue crew from Kingwood, West Virginia, equipped with breathing apparatus and equipment to bore a hole through the debris, was preparing to enter the mine, he said. "It's probably going to be a long, involved process," he said. Noting that there has been no communication with the trapped workers, he said, "There's no indication at this point whether they are alive or not."
Milligan identified the location as the Sago Mine, owned by the International Coal Group, an energy-consulting firm.
The miners, he said, were trapped 5,800 feet below the surface. (Map)
A rescue team made up of federal and state mine safety workers was on site, as were local fire, police and emergency medical service workers.
"It's probably going to be a long, involved process," he said. Noting that there has been no communication with the trapped workers, he said, "There's no indication at this point whether they are alive or not."
"There was some type of explosion either heard or felt by the miners attempting to go in the mine for a shift change," The Associated Press quoted Lara Ramsburg, spokeswoman for Gov. Joe Manchin, as saying. "They then backtracked out of the mine."
Milligan said the explosion was likely caused by methane gas. "I know they're sucking the bad air out; not sure if they're blowing fresh air in," he said.
According to the U.S. Department of Labor's Mine Safety and Health Administration, 14 underground fatalities occurred among miners in 2004, the last year for which data were available.
International Coal Group was formed in May 2004 when business entrepreneur Wilbur Ross Jr. led a group that bought many of the assets of Horizon Natural Resources in a bankruptcy auction.
Last March, ICG bought Anker, which previously owned Sago. ICG has mining operations in West Virginia, Kentucky and Maryland.
In February 2003, three contract workers were killed by a methane explosion while drilling an air shaft at a Consol Energy Mine near Cameron, West Virginia, according to AP. The state ended 2005 with three mining fatalities, the lowest since 2000, AP reported.
In July 2002, nine miners were rescued after being trapped for 77 hours in a mine near Somerset, Pennsylvania.