U.S. food safety regulators on Tuesday made public a warning letter to Eggo waffle maker Kellogg Co in which they said the company had not gone far enough in its efforts to address food safety violations at its Atlanta frozen food plant.

An October 2009 Atlanta plant inspection found bacteria contamination in multiple areas and sanitation violations such as improper handling of trash and food and improperly sanitized equipment, the FDA said.

The FDA's letter, dated January 27, comes after the Georgia Department of Agriculture found Listeria monocytogenes in Eggo Buttermilk Waffles on August 31.

Representatives from Kellogg were not immediately available for comment.

Kellogg's plant had "significant deviations" from the current manufacturing practices for food manufacturers and Kellogg's response so far had not addressed the violations, the agency wrote.

FDA inspectors found the presence of Listeria on five swabs from around the Atlanta plant. One of those positive swabs came from the wheels of a forklift used in the plant.

Listeria is a bacteria that can contaminate food and cause a mild illness or a severe, sometimes life-threatening, illness called invasive listeriosis, FDA said.

FDA said the company listed a number of corrective actions in its November 18 updated response to the agency, but said Kellogg needed to go further.

FDA said it is "essential" for Kellogg to identify all of the areas in its facility where Listeria can grow and survive and to take the actions necessary to control the organism. The regulator also recommended that Kellogg start a pathogen monitoring program.