At least four people were killed in a coal mine explosion in the border city of Piedras Negras in Mexico\'s northern Coahuila state, local authorities said Wednesday.The delegate of the Attorney General in the border region, Santos Vasquez Estrada, said miners were performing downhole operation when a gasbag exploded around 9:00 a.m. local time (1500 GMT), and four people were burned to death on the spot.The official said an investigation over the cause of the tragedy has begun but no details are available yet.