ROLETTE (AP) - Health officials believe they have solved the mystery of the contaminated water in this city, where residents have been under orders to boil their water before drinking it for the past two weeks.

"On June 7, we had test results that indicated there were three water samples (in Rolette) that had the presence of E. coli," said Jenifer Messer, a state Health Department environmental scientist.

Workers could find no water main breaks or contaminated wells in Rolette, Messer said.

But on Tuesday, Messer said, a crew discovered a broken screen on the town's water tower. Birds were nesting inside, she said.

"Right now, we are going to assume that this is what is causing the water problem," she said.

Crews plan to clean up and disinfect the tower and city water lines, which could take several days. Then, more tests will be done to see if the problem has cleared, Messer said.

She still recommends that residents boil water before using it to brush teeth or to cook.

E. coli-contaminated water in North Dakota happens "maybe once, twice a year," she said.