The U.S. Food and Drug Administration (FDA) sent warning letters to Walmart, Target, Kroger and Albertsons after investigators found recalled ByHeart infant formula still being sold or displayed well after a nationwide recall tied to an outbreak that has sickened more than 50 babies in 19 states.The letters, dated last week, demand explanations and corrective actions, give companies 15 working days to respond, and come as the Centers for Disease Control and Prevention expanded its count of infants treated for botulism linked to ByHeart formula while federal and state officials continue to trace the outbreak source.