WASHINGTON — Food and Drug Administration investigators have found rodents, seeping manure and even maggots at the Iowa egg farms believed to be responsible for as many as 1,500 cases of salmonella poisoning.

FDA officials released their initial observations of the investigations at Wright County Egg and Hillandale Farms on Monday. The two farms recalled more than half a billion eggs after salmonella illnesses were linked to their products this month.

The reports released by the FDA show many possible sources of contamination at both farms, including rodent, bug and wild bird infestation, uncontained manure, holes in walls and other problems that could have caused the outbreak. Positive samples of salmonella linked to the outbreak have been found at both farms.

Officials said they still cannot speculate about how the eggs were contaminated. But they said the farms not only violated their own standards but also new egg rules put in place this summer.

In a statement, unidentified officials of Wright County Egg said they are working “around the clock” to address FDA’s concerns and some have already been fixed.

“To date, the vast majority of the concerns identified in the FDA report already have been addressed through repairs or other corrective measures,” the company said. “We anticipate the expeditious completion of nearly all remaining items by mid-September.”

Hillandale Farms released a similar statement, saying some of the issues had already been identified by the company.

Animal feces and access to wildlife are normally the main concern of investigators looking for causes of an outbreak, as illnesses such as salmonella originate from feces. Michael Taylor, the FDA’s deputy commissioner for foods, said in a briefing for reporters Monday that the agency cannot say how these conditions compare to other egg farms around the country but he believes they are “significant deviations from what is expected.”

The agency has not traditionally inspected egg farms until there has been a problem. The FDA has said it has not inspected either of the two Iowa farms, even though the owners of Wright County Egg have a long history of health, safety, environmental and immigration violations.

The FDA will now inspect all of the nation’s largest farms by the end of next year, the Obama administration announced last week.

No deaths have been reported from the outbreak, but the number of illnesses, which can be life-threatening, especially to those with weakened immune systems, is expected to increase.

The CDC has said this is the largest outbreak of this strain of salmonella since the start of the agency’s surveillance of outbreaks in the late 1970s.