Two slicing machines at the Maple Leaf Foods plant in Toronto are believed to be the most likely source of listeriosis that led to a deadly outbreak, the company's CEO said Friday night.

During an investigation into the outbreak, "areas were found deep inside the machines where bacteria may have accumulated and avoided our rigorous sanitization procedures," Michael McCain told reporters.

The machines had been cleaned on a daily, weekly and monthly basis as per protocol, he said.

He pointed to the location of the service elevator, floor drain and tote bins as other possible contributing factors, even though those surfaces were not considered to have come in contact with the products.

McCain said the investigation's findings were not conclusive because the pervasiveness of listeria "makes an absolute determination [of the outbreak] impossible."

"When [the experts] complete their investigation, the best they can do is advise what their best judgment is, and they feel quite strongly about that judgment," he said.

"In their best judgment, this is the most likely cause."

There have been 13 deaths directly linked to listeriosis found in meat from the Toronto plant.

The investigation shut down the entire plant and all 220 products produced at the facility were recalled, at an estimated cost of about $20 million.

McCain said following the outbreak, the company removed all products and machinery from the plant. The plant has also undergone four sanitization cycles, he added.

The slicing equipment on the two affected product lines have been completely torn down and deep cleaned four times, tested multiple times and reassembled.

All remaining lines in the plant have also been deep cleaned and reassembled.

McCain said Friday he hopes consumers will accept an apology issued by the company in newspapers and TV ads last month and find a way to trust the company once again.

"That's a process that will certainly take time," he said. "I think it's going to be our consumers that determine whether they have confidence in us, not Maple Leaf."

Prime Minister Stephen Harper has promised an independent investigation into the outbreak.

With files from the Canadian Press