A nonprofit advocacy group has sued the U.S. Department of Agriculture, alleging the agency has not appropriately responded to their concerns about fecal contamination in the production of chicken and other meats.

The lawsuit was filed by the Physicians Committee for Responsible Medicine on Tuesday in U.S. District Court in Washington, D.C. The group says it has 12,000 members who are doctors; it advocates for "plant-based diets and ethical and effective scientific research."

The group has previously petitioned the USDA to change how it regulates fecal contamination in meat and poultry production, the lawsuit says.

That effort followed a 2011 study by the nonprofit which allegedly found fecal bacteria in 48% of analyzed poultry products sourced from 10 U.S. cities.

"Although USDA implements a 'zero tolerance' policy for fecal contamination, this policy applies to visible fecal contamination only," the lawsuit alleges "The result is that fecally contaminated meat and chicken products pass inspection as long as the feces on them are not visible to the naked eye."

Since the group petitioned the USDA in 2013, the situation has worsened, the lawsuit alleges, citing increased privatization of the inspection process.

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The lawsuit seeks a "substantive response" to that original petition. It also claims the agency has not adequately responded to a 2017 Freedom of Information Act request regarding fecal contamination rates and other data relating to poultry production.

The USDA responded Tuesday to the lawsuit by saying it "disagrees with the underlying assumption that meat and poultry products bearing the mark of inspection are likely to be contaminated with feces," the Washington Post reports.

The agency's response says pathogen testing is used to prevent fecal contamination, the newspaper reports. Proper cooking of meat also destroys pathogens, the agency said.

Previously published material from the North American Meat Institute says bacteria found on meat products is not the same as "fecal contamination."

"All raw agricultural products, whether bananas, beef or broccoli, contain bacteria," the industry group says.

Publications from The Physicians Committee for Responsible Medicine and the North American Meat Institute show the groups differ on how the presence of E. coli in meat should be presented to the public.

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Noting that only specific strains of E. coli are known to make people sick, the Meat Institute says: "E. coli is not considered an 'adulterant.' If it were an adulterant in food, most fresh food would need to be recalled."

The publication goes on to note, "A swab of phones and keyboards would likely find E. coli, but that doesn’t mean there is ‘poop’ on your phone."

In contrast, the Physicians Committee for Responsible Medicine has used the presence of E. coli as an indicator of fecal contamination, citing "USDA and industry testing."

The group has conducted pointed campaigns on the topic of fecal contamination in past years. In 2012, the group suggested chicken should be marked with a warning sticker saying “May Contain Feces.”