Blaine officials are telling residents to continue boiling their water until test results about water quality come back Monday afternoon.

Water samples were sent to the Minnesota Department of Health for testing after the city’s water system failed Sunday morning leaving thousands residents and businesses without water for much of the day and forced several schools to call off classes Monday.

Even though the water system now is up and running and no safety or contamination issues have been identified, city officials urged residents to follow the precaution of boiling their water until test results are returned. They are expected to be known by mid- to late-afternoon said city spokeswoman Sandy Biebl.

Anytime there is low water pressure, bacteria can enter the system, the city said.

The outage also affected some residents in neighboring Lexington, Anoka County officials said.

As for what caused the widespread outage, city officials are not totally sure. An initial review indicated that a “failure of software communication systems within the water delivery system” was to blame, a statement posted on the city’s website said.

The lack of water forced schools in the Spring Lake Park, Centennial and Anoka Hennepin school districts to give students an unexpected day off. In Spring Lake Park, Northpoint Elementary, Westwood Intermediate School and Westwood Middle School were closed Monday.

In the Anoka Hennepin school district, closed schools include Jefferson, Johnsville, University Avenue and Madison elementary schools, Roosevelt Middle School and Blaine High School. If a student goes to another school but gets picked up at school in Blaine, the bus will be there to transport them, the district said.

In Centennial, Centennial Elementary School and Centennial High School were closed and all before and after-school and evening activities were canceled, including the elementary school’s preschool. All other schools in the district were open as scheduled.

Residents woke up to a citywide water problem Sunday morning. Some had no water; others had a trickle, said Bob Therres, Public Works Department manager. Utility workers searched without success for a physical problem with the system, but will continue looking into the situation Monday.