U.S. health officials are investigating whether a popular powdered baby formula fed to a 10-day-old boy triggered a bacterial infection that killed him.

The world's largest retailer, Wal-Mart Stores, pulled thousands of cans of the product, Enfamil Newborn, from its U.S. store shelves this week after the infant's death in the central state of Missouri. Other large retailers also stopped selling the product. The infant tested positive for Cronobacter, a bacteria that sometimes has been found in powdered baby formula and can cause serious illness or death.

But Enfamil's manufacturer, Mead Johnson Nutrition, said the batch of the product used by the child's family did not show any bacteria when it was produced and packaged. The company, whose stock value fell 10 percent Thursday as news of the investigation spread, did not recall the product.

Health officials are also investigating the distilled water the baby's family used to reconstitute the formula and other factors that may have caused the bacterial infection. The test results are not expected until the middle of next week.