U.S. federal health officials, criticized for declaring that a controversial chemical is safe, have refused to back down and instead plan more research.

The Food and Drug Administration, in a letter to independent scientific reviewers released Monday, said it is re-evaluating its research on bisphenol A and carrying out additional studies. The letter was the agency's initial response to an independent report that found deep flaws in the FDA's conclusion that the chemical — used in food packaging — is safe.

Known as BPA, bisphenol A is used for hardening plastics that go into all sorts of consumer products. The canning industry uses it for coatings that prevent leaks and bacterial contamination in metal food containers. But Canada has banned it in baby bottles, because of concern it could cause developmental problems in infants. BPA mimics some of the effects of estrogen, a powerful hormone.

"FDA is re-evaluating available data, and planning for the acquisition of additional data that will strengthen the exposure estimates from all dietary sources of BPA, with particular attention to dietary sources relevant to infants and children," the agency's letter said.

Urvashi Rangan, a senior scientist with Consumers Union, said that while additional research would be useful, "the big, noteworthy part is what's missing from this letter, which is the lack of any FDA action while it investigates serious health concerns."