McDonald’s responded earlier this week by temporarily removing Quarter Pounders and fresh slivered onions from about a fifth of its stores in the US.

Investigators are eyeing the possibility that the onions, a popular topping, could be the cause of contamination.

The CDC and FDA said on Tuesday that they had not yet ruled out that the patties themselves could be to blame.

The chain has agreed with that assessment, but also said that its burgers are cooked to 175 degrees – which is above the 160 degree level needed to kill the E. coli bacteria

The cases involve purchases from multiple stores, making it unlikely that food preparation is at fault, McDonald’s said.

The company said the stores involved had also used multiple suppliers for the beef patties, but shared a single supplier of onions – identified as California-based Taylor Farms, one of the world’s largest vegetable processors.

Taylor Farms, which works with major food suppliers such as US Foods, has issued its own recall of some batches of onions out of an abundance of caution.

Taylor Farms said in a statement provided to the BBC’s US news partner, CBS News, that it conducted tests on “raw and finished” onion products and “found no traces of E. coli”.

The CDC said on Friday that due to the product actions taken by McDonald’s and Taylor Farms it believed “the risk to the public is very low”.