The Winnipeg Regional Health Authority has lifted its boil water advisory for city residents after tests of water samples came up negative for any trace of E. coli.

Winnipeg Mayor Brian Bowman made the announcement Thursday.

"Our city's drinking water remains safe to drink, prepare food and beverages, along with all hygiene practices," Bowman said.

After making brief remarks, Bowman took a gulp of tap water from a glass he was carrying.

Diane Sacher, director of the city's water and waste department, said she will launch an investigation into how the city's water supply turned up low levels of E. coli.

"That’s obviously going to be a high priority for us right now, to try to find out why that happened and find out what we can do to prevent that from happening again," Sacher told reporters.

The city issued the boil advisory on Tuesday, after very low levels of E. coli were detected in six of 39 water samples. Residents were urged to bring water to a rolling boil for at least one minute before it was used for drinking, cooking, brushing teeth, making ice or preparing infant formula.

The situation was exacerbated Wednesday for some 9,000 residents who lost power in their homes after a fire affected a power line and were unable to boil water.

Schools banned the use of drinking water, while hospitals brought in extra bottled water for patients. Grocery and drug stores quickly sold out of bottled water.

Water samples tested on Wednesday were negative for the bacteria. A second round of testing confirmed those results, Bowman said.