"The feeling is that it's a common source across all three provinces," said Dr. Frank Atherton, Nova Scotia's deputy chief medical officer.
The fast-food outlets are not believed to be the responsible for the contamination, and neither is the Ontario-based firm that distributed the lettuce, he added.
Atherton said KFC and Taco Bell outlets have been inspected, but he didn't know how many.
"We don't believe there's a problem in the KFC/Taco Bell outlets themselves," he said. "The problem is really in the food that's been distributed."
In a statement, KFC Canada said it has removed all the affected lettuce from its restaurants and reassured its customers that food served at its outlets is safe to eat.
Atherton said it's unlikely that more E. coli cases will be reported because the lettuce in question has passed its shelf-life and is probably out of the food chain.
"We're not seeing new cases," he said, noting that the most recent case in Nova Scotia was recorded Jan. 2.
"If there was infected material still being served in restaurants we would expect to continue to see cases. At the moment, we're not, but we remain vigilant."
As an added precaution, the Canadian Food Inspection Agency is recalling the lettuce believed to be at the centre of the outbreak.
Atherton said none of the lettuce in question was shipped to grocery stores.
The federal government is continuing to investigate the cause of the contamination, which means inspectors are now looking farther up the food chain at farms.
New Brunswick has reported six cases of the infection, Nova Scotia has had 10 and there have been at least 10 cases in Ontario.
Ontario's associate chief medical officer, Dr. Robin Williams, issued a statement saying the Public Health Agency of Canada had confirmed the lettuce in question was distributed by FreshPoint Inc., a Houston-based company that describes itself as North America's largest produce distributor.
Company spokesman Charley Wilson said FreshPoint is waiting for a report from the Canadian Food Inspection Agency.
"FreshPoint has reviewed its supply sources associated with this investigation of contaminated produce and we believe that adequate food safety controls are in place with our suppliers to ensure that this will not recur," he said.
Wilson said he couldn't say where the lettuce came from until federal officials had completed their work.
Note to readers: This is a corrected story. A previous version said five of 10 cases in Nova Scotia were linked to the outbreak.