Slivered onions are the likely source of the multi-state E. coli outbreak linked to McDonald's Quarter Pounder burgers that continues to grow, the Centers for Disease Control and Prevention announced Wednesday.

Onions were one of two primary suspects when the CDC announced the outbreak on October 22, with the other being the beef patties used on the burgers. But onions quickly became the leading suspect. The day after the CDC's announcement, McDonald's onion supplier, Taylor Farms, recalled peeled and diced yellow onion products and several other fast food chains took onions off the menu as a precaution. (No other restaurants have been linked to the outbreak to date.)

According to the CDC, traceback information and epidemiological data collected since then have all pointed to the onions and, according to McDonald's, state and federal testing of the beef patties has all come back negative.

In the CDC's update Wednesday, the agency reported that 15 more people were identified as sickened in the outbreak, including five who were hospitalized. In all, that brings the outbreak to 90 cases, including 27 hospitalizations and one death, which collectively span 13 states.

All the newly reported illnesses had onsets prior to the October 23 onion recall. The most recent illness onset was October 16. Additional illnesses may be reported, as it can take three to four weeks to link illnesses to an outbreak.

"Due to the product actions taken by McDonald’s and Taylor Farms, the CDC believes the continued risk to the public is very low," the agency said in a media alert.

McDonald's says that Quarter Pounders—without onions—will return to the menus of affected restaurants this week. Prior to the recall, 900 restaurants had received onions from Taylor Farms, including in Colorado, Kansas, and Wyoming, as well as portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, Oklahoma, and Utah.