CHARLESTON, W.Va. - Federal regulators temporarily closed six underground coal mines in Kentucky after turning up hundreds of violations during a mid-April inspection blitz following an explosion that killed 29 miners in West Virginia.

The Mine Safety and Health Administration said Thursday it has since sued two of the operations for tipping off workers that federal inspectors were on the property.

Overall, MSHA says it issued more than 1,300 citations during the five-day blitz. Massey Energy owns the Upper Big Branch mine in West Virginia where the April 5 explosion occurred.

MSHA's preliminary report lists methane and coal dust as the cause. The agency focused the blitz on rules covering methane, ventilation and efforts to control coal dust.