Twenty-five miners were killed Monday after a coal mine explosion deep in the rugged hills of West Virginia, marking the worst U.S. mine disaster since 1984 and raising questions about safety precautions at one of the nation's largest coal producers. The search for survivors at the Upper Big Branch mine [operated by Massey subsidiary Performance Coal Co.] was called off early Tuesday...according to...Massey Energy Co., the mine owner...Officials said they didn't yet know the cause of the accident. But federal mine safety and health officials have repeatedly cited Massey...for potentially dangerous conditions...In the last year, federal inspectors have fined the company more than $382,000 for repeated serious violations...Massey Energy says on its website that it has an above-average safety record, with three fatalities at the Upper Big Branch mine in the last 12 years.