The Canadian Food Inspection Agency couldn't determine the source of E. coli contamination in burgers that were recalled in mid-December.

The frozen beef burgers were recalled Dec. 12 to 15 after a cluster of illnesses in Ontario and Alberta.

The CFIA investigated three main items in the burgers — spices, domestic beef ingredients and international beef ingredients.

On Dec. 21, the CFIA said spices were determined not to be the source of contamination and all international ingredients met import certification.

In its further investigation of the domestic beef, the agency assessed production, inspection and testing records, reviewed plant safety procedures and conducted additional tests on domestic beef ingredients.

"All available domestic beef ingredient products have tested negative for E. coli O157:H7," the agency said.

The investigation has concluded, and a detailed report will be released in the coming weeks.