U.S. food safety regulators have warned Whole Foods Market Inc that inspections at the grocer's Everett, Massachusetts, plant turned up violations that could result in food being "contaminated with filth or rendered injurious to health."

Shares in Whole Foods, which has been working to bolster sales amid intense competition, were down 2.6 percent to $32.61 in afternoon trading on Tuesday.

During five inspections between Feb. 10 and 26, inspectors found food products that were prepared, packed, or held under insanitary conditions, the U.S. Food and Drug Administration said in a warning letter dated June 8 and posted on its website.

The agency said in the letter that it had received a response from Whole Foods outlining steps it would take to correct the issues.

"We do not consider your response acceptable because you failed to provide documentation for our review, which demonstrates that all your noted corrective actions have been effectively implemented," the FDA said.

The FDA gave Whole Foods 15 days from the receipt of the letter to document the corrective actions it has taken, or to state the reasons for any delays.