An investigation continues into E. coli contamination of dozens of brands of burgers and steakettes produced at a single plant in Saskatoon, but the source remains a mystery.

Though 135 potentially contaminated brands have already been recalled, the Canadian Food Inspection Agency says that list could grow as their investigation continues.

CFIA investigators appear to feel confident that the E. coli that has sickened at least one person in Alberta can be traced back to products from the Saskatoon processing plant of hamburger patty maker New Food Classics.

That company is now in receivership and both their Saskatoon and St. Catharines, Ont. facilities are no longer in operation.

As for how burgers from the facility became contaminated with E. coli, it appears that investigators are not yet sure. The CFIA says the possibly contaminated products were produced as far back as July, 2011. The fact that investigators are looking that far back and the list of recalled products is still growing suggests that investigators are still looking into the history of the contamination.

The CFIA has brought in Health Canada, and Public Health Agency of Canada as part of the probe.

The agency says it expanded the burger recall on Mar. 17 to include all ground beef products from the Saskatoon facility manufactured between July 1, 2011 and February 15, 2012 after reviewing Health Canada's health risk assessment.

"Since that time, the CFIA has been working continuously with the responsible company to gather detailed product information," the CFIA said on Tuesday.

E. coli often enters the food supply through beef. Keith Warriner, an associate professor of food microbiology at the University of Guelph says all beef cattle can carry E. coli 0157:H7 harmlessly in their intestines without suffering the symptoms of E. coli infection.

"About 10 per cent of the cattle in North America carry it," he told CTV's Canada AM Wednesday.

"During processing, E. coli in the manure – if you want to say it politely – gets on the carcass and then we grind up the meat and sure enough, we have E. coli in our meat," he explained.

But Warriner notes that less than 1 per cent of the ground beef patties in stores at any given time has E. coli in it.

"So I think a lot of the recall we have at the moment is more precautionary – some people say an overreaction," he said.

Investigators are likely showing an abundance of caution because of the dangerous nature of E. coli 0157:H7 and the serious health consequences that can result from consuming the bacteria.

The strain is the same one that led to the deaths of seven people and the illnesses of 2,500 more in Walkerton, Ont., in 2000.

E. coli 0157:H7 can cause severe -- and often -- fatal food poisoning. Symptoms usually begin within three to four days of eating contaminated food and the illness can last between five to 10 days.

Even those who survive the illness endure include severe abdominal pain and bloody diarrhea. The infection can lead to significant blood loss and permanent kidney damage and is fatal in many cases.

Warriner says that cooking food to a safe internal temperature is effective at killing E. coli in ground beef. The problem, he says, is many people judge whether a burger is cooked by looking at the colour of the meat.

That is highly ineffective method for checking for "doneness," Warriner says. Instead, it's best to use a meat thermometer and check for an internal temperature of 74 degrees Celsius (165 F).

"The other risk is from handling the raw hamburger, because we can touch it and transfer the bacteria to our hands and then to salads and get cross-contamination," Warriner said.

Perhaps because of the known risk of cross-contamination, the CFIA is advising the public in this recall not to consume the affected products at all.

"Our message to consumers is if you have these products in your freezers, do not consume them," CFIA food safety and recall specialist Garfield Balsom told CTV News Tuesday.

Balsom said the CFIA learned of the E. coli-related illness on Feb. 15 and issued a recall of two beef products on Feb. 18.

"The investigation continued into that lot of product and culminated in the recall you're seeing (now)," Balsom said.

While there have been a number of E. coli infection outbreaks in recent years, the source of many of them is never clear.

An outbreak of E. coli linked to spinach in 2006 sickened at least 276 people in the U.S. and led to the deaths of three people. Investigators determined that outbreak was likely caused by manure runoff from an Angus cattle ranch that had leased land to the grower of the contaminated spinach.

But in their final report, U.S. investigators said they could not make a "definitive determination" as to how the E. coli contaminated the spinach.

A full list of the products affect in this latest recall is available on the CFIA website.