Authorities in the Canadian city of Winnipeg said Thursday that the tap water was once again safe to drink, bringing to an end a 48-hour boil-water advisory.

Residents in Canada's sixth-largest city had been told Tuesday to boil after tests showed E. coli in six of 39 samples of city water. The bacteria can cause stomach upsets.

But new samples tested negative for E. Coli, and the water was deemed safe to drink.

"I'd like to thank all Winnipeggers for their patience over the last two days," Mayor Brian Bowman said.

Officials were still trying to figure out what had caused the contamination. Winnipeg has a population of about 700,000.