TORONTO (Reuters) - The death toll from an outbreak of deadly E. coli bacteria that swept through a small Ontario town last month may be raised to 18, Canadian health officials said on Tuesday.

Health officials said they were investigating four more deaths that could be linked to the E. coli outbreak in Walkerton, Ontario, a farming community of 5,000 about 200 kilometres (125 miles) northwest of Toronto.

Ontario's chief coroner said four more deaths had been reported to the provincial police, raising the total number of possible E. coli-related deaths in the Walkerton area to 18.

Police added the deaths of an elderly man and woman as well as a 25-year-old man in Walkerton to their investigation. The death of an elderly man in nearby Hanover is also being looked into.

Health authorities have confirmed that at least seven people died from water contaminated by E. coli bacteria during the epidemic, which began in late May and affected about 2,000 people in and around Walkerton.

E. coli (Escherichia coli) is a common bacterium usually found in the intestines of humans and animals. Some strains--like the often lethal 0157 variety that hit Walkerton--can cause dangerous, even life-threatening, infections.

Officials do not yet know exactly how the deadly bug got into Walkerton's water system but suspect heavy rains may have washed infected manure into the farming community's wells.