Federal inspectors turned up more than 60 serious safety violations at Massey Energy operations after the explosion that killed 29 miners at the company's Upper Big Branch mine, federal mine safety records show.

Inspectors visited more than 30 underground Massey mines in West Virginia, Kentucky and Virginia after the April 5 blast, according to records from the Mine Safety and Health Administration. The agency has tentatively blamed preventable accumulations of explosive methane gas and coal dust for the worst U.S. coal mining disaster since 1970.

The violations include conveyer belt problems at Massey's Aracoma Alma No. 1 mine in West Virginia, where a belt fire killed two men in 2006. The company's Solid Energy No. 1 mine in Kentucky was cited for allowing coal dust to pile up on three occasions since the explosion. Massey's Mammoth No. 2 Gas mine near Charleston was cited after a spot MSHA check turned up a crew without a methane monitor April 7.

Mines are required to keep methane well below explosive levels with sophisticated ventilation systems and control coal dust by keeping it from piling up and covering it with noncombustible material.