The FDA posted warning letters after telling Walmart, Target, Kroger and Albertsons they failed to promptly remove recalled ByHeart infant formula tied to a botulism outbreak, following earlier findings that the product remained for sale after the recall.Federal investigators documented the formula on store shelves in many states for days or weeks after the mid-November recall and say the outbreak has sickened more than 50 infants in 19 states who were hospitalized and treated.Retailers say they blocked sales and removed stock but have 15 working days to respond to the FDA letters, and Steven Mandernach of the Association of Food and Drug Officials criticized delays in sharing product lists with state and local officials.