E. coli contamination prompts order to boil water in Portland, Ore.

About 670,000 people in the Portland, Ore., area were told Friday to boil their water after three tests showed the system was contaminated by E. coli bacteria, usually associated with animal or human fecal matter.

The emergency notice is the city’s largest, and officials said tap water in the affected areas should be heated to a full, roiling boil for at least one minute. Portland, whose population is just over 600,000, supplies about 935,000 customers in the metropolitan region.

“While we believe at this time that the potential health risk is relatively small, we take any contamination seriously and are taking every precaution to protect public health,” Portland Water Bureau Administrator David Shaff said in a statement.

Routine inspections at two of the city’s Mt. Tabor reservoirs produced the three positive E. coli tests. Those results require a boil notice under state law, Portland City Commissioner Nick Fish said at an afternoon news conference.

The next set of tests will be available Saturday morning. Officials said they will use those results to decide whether to lift the boil notice.

“Our history is that this is usually a one-day, transient event,” Shaff told reporters. “I have every expectation that we will declare an all-clear tomorrow.”

Officials said they will investigate how the water became contaminated.

In an unrelated event last month, city officials for the second time in three years emptied a reservoir amid concerns that a teenager had urinated in the water. Tests showed it was safe to drink the 38 million gallons of water, which was diverted to another reservoir where it was used as part of an experiment in monitoring how long water stays fresh.

Dallas Swonger, 19, was caught on camera in April appearing to urinate into the reservoir. Swonger was cited for public urination and trespassing and insisted to KATU-TV that he urinated against a wall, not into the water.