MERCER ISLAND, Wash. (AP) — Residents of a wealthy Seattle suburb were told to boil their drinking water through the weekend despite 15 new water samples testing clean of E. coli contamination Friday.

It was the second time in a week people were ordered to take precautions with tap water in Mercer Island, a city of 24,000 on an island in Lake Washington whose residents include billionaire Microsoft co-founder Paul Allen. The warning is unusual because most boil-water advisories hit small communities in rural areas.