Some of the nation's biggest retailers are receiving stern warning letters after regulators found recalled baby formula still being sold weeks after it was pulled for safety concerns.

The U.S. Food and Drug Administration sent warning letters to Walmart, Target, Kroger and Albertsons, saying investigators found that stores continued selling recalled infant formula tied to a growing botulism outbreak.

The product, ByHeart Whole Nutrition Infant Formula, had been linked to at least 51 suspected or confirmed cases of infant botulism in 19 states as of Dec. 10, federal officials reported.

The FDA said retailers were first notified of the recall Nov. 8, after health officials determined the formula was the likely source of the outbreak.

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