McDonald’s has stopped selling Quarter Pounder hamburgers at about one-fifth of its U.S. restaurants as federal health officials investigate an outbreak of E. coli that has sickened nearly 50 people in 10 states, killing one.

The fast-food chain sold about 1 million Quarter Pounders in the timeframe that the illnesses occurred, a McDonald’s spokesperson said Wednesday. The company said it believes more people would have been sickened if the contamination stemmed from the patties themselves, adding that onions used on the burgers may be the culprit.

Burgers sold by McDonald’s are cooked at 175 degrees, above the 160 degree level needed to kill the E. coli bacteria, according to the spokesperson. 

However, slivered onions used as a topping on Quarter Pounder burgers sold in the impacted region were not cooked and came from a single supplier. “If that is the source, it will be the first time onions have been a carrier for this strain of E. coli,” the spokesperson said.

The U.S. Centers for Disease Control and Prevention on Tuesday linked McDonald’s Quarter Pounder hamburgers as being linked to an outbreak of E. coli that has sickened dozens of people, mostly in Colorado and Nebraska.

Washington — Police have identified a suspect arrested in connection with three shootings targeting a Democratic National Committee office in Tempe, Arizona, since September. 

Jeffrey Michael Kelly, 60, was also arrested for allegedly hanging suspicious bags of white power from political signs lined with razor blades in the nearby village of Ahwatukee, according to the Tempe Police Department. 

Police announced the arrest Tuesday. Kelly is charged with several felonies, including committing an act of terrorism, unlawful discharge of a firearm and shooting at a non-residential structure. He is also charged with criminal damage, a misdemeanor. 

After the first two shooting incidents in September, police said there were gunshots through the office’s front windows. The first incident on Sept. 16, police said, appeared to be from a BB gun or pellet gun. The second incident occurred Sept. 23. 

“No one was inside the office during the overnight hours, but this raises concerns about the safety of those who work in that building, as well as those nearby,” Sgt. Ryan Cook, the police department’s public information officer, said in a Sept. 24 statement. 

The third incident occurred on Oct. 6, according to azfamily.com. 

The office was shared with staff from Vice President Kamala Harris‘ campaign.