Contaminated Spanish onions are being blamed for an E. coli 157:H7 outbreak last fall that sickened more than 235 people in Ontario.

The North Bay Parry Sound District Health Unit released a 103-page investigative summary of the outbreak, which was linked to a Harvey’s fast food restaurant in North Bay, Ontario. Inconsistent cleaning of the onion dicer may have perpetuated the contamination for several days, according to the report.

“However, we were not able to confirm how they (onions) became contaminated,” said Dr. Jim Chirico, acting medical officer of health, in a news release.

Results of environmental testing, food sampling, trace back investigation and testing of the restaurant’s employees did not confirm how the onions were initially contaminated.

The Harvey’s restaurant was closed for a month before reopening Nov. 12.

The health unit worked with the Public Health Agency of Canada’s Canadian Field Epidemiology Program to conduct the investigation. The results were released June 22.