Prime Minister Stephen Harper has set the terms of reference for an investigation into the recent listeriosis outbreak linked to processed meat products that is blamed for the deaths of 13 people so far.

The announcement by Harper's office Saturday comes a day before the expected start of a federal election campaign.

The probe is to:

• Examine the events, circumstances and factors that contributed to the outbreak.
• Review the efficiency and effectiveness of the response by federal agencies in terms of prevention, the recall of contaminated products, and collaboration and communication among partners in the food safety system and the public.
• Make recommendations aimed at enhancing prevention of future outbreaks and the removal of contaminated products from stores and warehouses.

Investigators will not express any conclusion or recommendation about civil or criminal liability of individuals or organizations. Their report is due before next March 15.

The outbreak has been traced to a Maple Leaf Foods plant in Toronto. All 220 meat products produced at the facility were recalled and the plant was shut down while company and health officials tried to determine the source of the contamination.

On Friday, Maple Leaf CEO Michael McCain said two slicing machines are believed to be the most likely source of the listeria bacteria.

During an investigation, "areas were found deep inside the machines where bacteria may have accumulated and avoided our rigorous sanitization procedures," he said.

The outbreak has led to 38 confirmed cases of listeriosis across the country and 20 more suspected cases.