Health authorities tested water for toxins in Toledo, Ohio, on Sunday as some 400,000 people remained without safe drinking water for a second day following the discovery of high toxin levels from algae on Lake Erie.
Toledo Mayor D. Michael Collins said some sampling showed decreased toxin levels but results from further tests would not be known until later in the day.
Health officials sent samples to several laboratories after finding Lake Erie, which provides the bulk of the area's drinking water, may have been affected by a "harmful algal bloom," Ohio Environmental Protection Agency spokeswoman Heidi Griesmer said.
Ohio Governor John Kasich declared a state of emergency on Saturday for the state's fourth-largest city and surrounding counties. The city and other agencies have established sites where bottled water is being distributed free to the public.
Many residents drove to other states in search of fresh water as stores rapidly sold out of bottled water.
Algal blooms in Lake Erie are fairly common, typically in the summer, state emergency operations spokesman Chris Abbruzzese said.
Potentially dangerous algal blooms, or rapid increases in algae levels, are caused by high amounts of nitrogen and phosphorous.
Those nutrients can come from runoff of excessively fertilized fields and lawns or from malfunctioning septic systems or livestock pens, city officials said.
Drinking the contaminated water can affect the liver and cause diarrhea, nausea, numbness or dizziness, officials said. Boiling will not destroy the toxins.