The US FDA is investigating whether the onions in McDonald’s Quarter Pounder burgers are the source of the outbreak.

Many people in Colorado and around the country became ill following an E. coli outbreak tied to McDonald’s Quarter Pounders.

A California-based produce company has been identified as the source of the fresh onions linked to the McDonald’s E. coli outbreak, and the company has issued a voluntary recall of onion products from a Colorado distribution center.