Colorado has seemingly eliminated one ingredient as a cause for death and illness, as states continue to investigate the source of an E. coli outbreak involving the Quarter Pounder hamburger at dozens of McDonald’s locations. As a result, the Quarter Pounder will begin to return to certain locations.

The Colorado Department of Agriculture announced McDonald’s brand “fresh and frozen beef patties” tested negative for E. coli after its lab analyzed dozens of subsamples.

CDA says it has completed all beef testing and does not anticipate receiving further samples.

Meanwhile, the federal investigation into the deadly E. coli outbreak in Colorado has focused on ground beef patties and onions. There continues to be no evidence that onions grown in Colorado are linked to the outbreak.

According to McDonald’s, The 900 restaurants that historically received slivered onions from Taylor Farms’ Colorado Springs facility will resume sales of Quarter Pounders without slivered onions. Those restaurants are in Colorado, Kansas, and Wyoming, as well as portions of Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico, Oklahoma and Utah. The restaurant chain also noted it has stopped getting onions from that facility indefinitely.

“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,” McDonald’s North American Chief Supply Chain Officer Cesar Piña said in a statement Sunday

Since the outbreak was first announced, CBS News Colorado confirmed one older man on the Western Slope died after consuming a Quarter Pounder from a McDonald’s location in the state. Initial information also confirmed more than two dozen people had become ill due to E. coli-affected Quarter Pounders.

Nationwide, this outbreak has sickened 75 people in more than a dozen states, but Colorado remains the only state impacted that has experienced a death due to it. 

The Colorado Department of Public Health says there have been 26 cases reported in nine different Colorado counties, and they are located in several different parts of the state:

• Arapahoe County
• Chaffee County
• El Paso County
• Gunnison County
• Larimer County
• Mesa County
• Routt County
• Teller County
• Weld County

The illnesses were reported between the last days of September and through Oct. 11. An investigation by the Centers for Disease Control and Prevention into the outbreak is ongoing.

McDonald’s company leaders previously said they’ve taken Quarter Pounders off the menu in states where there have been outbreaks.