The Food and Drug Administration said 90 people across 13 states have fallen ill in the outbreak, up from 75 at the end of last week. The number of people hospitalized increased by five, to 27 people. One death has been tied to the outbreak.

Officials have said raw, slivered onions on McDonald’s Quarter Pounder hamburgers are the likely source of the E. coli. McDonald’s said the onions came from a single supplier, the California-based Taylor Farms. The company has since recalled the yellow onions it sent to McDonald’s and other restaurant chains.

The FDA said Wednesday it has begun inspecting Taylor Farm’s processing center in Colorado Springs as well as an “onion grower of interest” in Washington state. It did not name the grower.

More than 80% of people with E. coli interviewed by government investigators reported eating McDonald’s items containing fresh, slivered onions, the FDA noted.

McDonald’s said last week that onions from the Colorado Springs facility were distributed to approximately 900 of its restaurants, including some in transportation hubs like airports. The restaurant chain announced that it would resume serving Quarter Pounders at hundreds of its restaurants after testing ruled out beef patties as the source of the outbreak.

The strain of E. coli behind the outbreak can cause dangerous diarrhea and lead to kidney failure and other serious problems. It causes about 74,000 infections in the U.S. annually, leading to more than 2,000 hospitalizations and 61 deaths each year, according to the Centers for Disease Control and Prevention.

FDA officials said Wednesday the current risk of illness “is low because the onions have been recalled and should no longer be available.” They noted that all of the reported cases occurred before Taylor Farms and McDonald’s removed the onions from the market.