McDonald’s Quarter Pounder burgers will return to roughly 900 restaurants this week after the fast-food giant pulled the menu item linked to a deadly E. coli outbreak.

Affected restaurants — roughly a fifth of the company’s U.S. footprint — will be serving the Quarter Pounder burgers without slivered onions for the foreseeable future as health authorities continue their investigation into the source of the outbreak. That change will affect restaurants in Colorado, Kansas and Wyoming and portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, Oklahoma and Utah.

“The issue appears to be contained to a particular ingredient and geography, and we remain very confident that any contaminated product related to this outbreak has been removed from our supply chain and is out of all McDonald’s restaurants,” Cesar Pina, chief supply chain officer for McDonald’s North American operations, said in a letter sent to the company’s U.S. system.

The Colorado Department of Agriculture’s testing did not detect E. coli in samples of the beef patties taken from restaurants in the area, according to Pina. The agency isn’t planning further tests of the company’s beef.

Instead, health authorities have honed in on slivered onions used in the Quarter Pounders as the likely suspect for the outbreak. The Food and Drug Administration is still investigating if onions produced by Taylor Farms are responsible. McDonald’s has stopped using Taylor Farms as a supplier for the ingredient indefinitely.

McDonald’s is now asking its beef suppliers to produce a new supply of the fresh beef patties used in its Quarter Pounders, Pina wrote in a letter sent to the company’s U.S. system. Customers can expect to see the menu item back in all restaurants in the coming week, although it will happen on a rolling basis, depending on delivery and resupply operations.

The Centers for Disease Control and Prevention said Friday that the E. coli outbreak linked to McDonald’s has led to 75 cases across 13 states. Out of 61 patients with information available, 22 have been hospitalized, and two people have developed a serious condition that can cause kidney failure, called hemolytic uremic syndrome. The agency also said previously that an older adult in Colorado died.

Based on reported cases so far, the outbreak took place between Sept. 27 and Oct. 11. Over a two-week period, McDonald’s typically sells roughly one million Quarter Pounders in the affected region, according to company spokespeople.

McDonald’s USA President Joe Erlinger apologized to customers who are feeling “ill, scared or uncertain” in a video posted on the company’s website.

“On behalf of the McDonald’s system, I want you to hear from me: we are sorry,” he said.

McDonald’s is expected to report its third-quarter earnings before the bell on Tuesday. Shares of the company have fallen 7% since the CDC linked the E. coli outbreak to its restaurants.