CBC News reports the boil water advisory was lifted more than 24 hours ago, but questions remain over how the samples at the heart of the Winnipeg-wide scare could’ve tested positive for bacteria and E. coli in the first place.

Six routine water samples taken on Monday showed the presence of bacteria, and E. Coli in some cases. After resampling and retesting, samples came up negative and the city lifted the advisory Thursday.

Mayor Brian Bowman assured Winnipeggers tap water was safe, and always had been, as the original tests were proven to be false positives.

Rick Holley, a professor at the University of Manitoba and expert in food safety, said that while mistakes like this do happen, they are unacceptable when hundreds of thousands of lives may be impacted.

“I still had concerns at that time and still do that the false positives might not be scientifically discredited,” said Holley. “It’s all too easy to continue testing until you get the results you want and any results you don’t want you discard as being false. That’s inappropriate.”

Holley said the only way to be sure Winnipeg water is safe is to understand what caused the positive results earlier this week.

“Why were those six samples positive? There has to be a reason why and that has to be established,” said Holley.

One telling detail released by the city was that all of the samples that tested positive were handled by the same employee.

The city provided a list of possible explanations for how the tests came to be positive, including:

• A contaminated water tap or container
• The water being contaminated during sampling
• Mistakes made at the lab during analysis

Holley said while he hopes the city provides more detailed answers and soon, he remains concerned about the city’s water monitoring procedures.