CHARLESTON, W.Va. -- Federal regulators have notified four mine operators that they have a potential pattern of violations and will face more scrutiny.The Mine Safety and Health Administration says warning letters were sent to two West Virginia mines -- Ten-Mile Coal Co. Inc.'s No. 4 Mine in Harrison County and Argus Energy's Deep Mine No. 8 in Wayne County. Letters also were sent to Pike Floyd Mining Inc.'s No. 3 Mine in Pike County, Ky., and Noranda Alumina's Gramercy Facility in St. James County, La.Operators of two inactive mines in Kentucky and Idaho also were notified in case they return to active status.It's the third round of screening since MSHA established the current criteria and procedures in 2010.