The U.S. Food and Drug Administration sent warning letters to retailers after investigators found they continued selling recalled infant formula.

The FDA has sent warning letters to four major retailers for continuing to sell baby formula linked to a nationwide outbreak of bacterial illness in infants.

The agency sent letters to leaders at the companies, saying they continued to sell ByHeart infant formula for days or weeks after the recall.

The U.S. Food and Drug Administration sent warning letters to retailers after investigators found they continued selling recalled infant formula.